example or examples keywords in jsonSchema is causing ajv validation to fail

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## Steps to reproduce

Define a model w/ property values samples expressed in jsonSchema as follow :
`import {Entity, model, property} from '@loopback/repository';

@model()
export class Basic extends Entity {
@property({
jsonSchema: {
type: 'string',
example: 'simple',
// examples: [{'simple-sample': {value: 'simple'}}],
},
required: true,
})
name: string;

@property({
type: 'string',
id: true,
generated: true,
})
id?: string;

constructor(data?: Partial) {
super(data);
}
}

export interface BasicRelations {
// describe navigational properties here
}

export type BasicWithRelations = Basic & BasicRelations;`

Reference this entity as the request body in a route :

` @post('/basics')
@response(200, {
description: 'Basic model instance',
content: {'application/json': {schema: getModelSchemaRef(Basic)}},
})
async create(
@requestBody({
content: {
'application/json': {
schema: getModelSchemaRef(Basic, {
title: 'NewBasic',
}),
},
},
})
basic: Basic,
): Promise {
return this.basicRepository.create(basic);
}`

## Current Behavior

Ajv validation fail when trying to call the route
w/ DEBUG='loopback:rest:validation'

` BasicController
loopback:rest:validation Request body schema: { '$ref': '#/components/schemas/NewBasic' } +0ms
loopback:rest:validation referencing: {
title: 'NewBasic',
type: 'object',
description: "(tsType: Basic, schemaOptions: { title: 'NewBasic' })",
properties: {
name: { type: 'string', example: 'simple' },
id: { type: 'string' }
},
required: [ 'name' ],
additionalProperties: false,
'x-typescript-type': 'Basic'
} +1ms
loopback:rest:validation Converted OpenAPI schema to JSON schema: { '$ref': '#/components/schemas/NewBasic' } +102ms

Request POST /basics failed with status code 500. Error: strict mode: unknown keyword: "example"
at checkStrictMode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/util.ts:211:28)
at checkUnknownRules (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/util.ts:27:22)
at Object.alwaysValidSchema (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/util.ts:17:3)
at /home/fgeslin/simple-lb4-app/node_modules/ajv/lib/vocabularies/applicator/properties.ts:23:48
at Array.filter ()
at Object.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/vocabularies/applicator/properties.ts:23:33)
at keywordCode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:523:9)
at /home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:265:9
at CodeGen.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:525:33)
at CodeGen.block (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:680:20)
at iterateKeywords (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:262:7)
at groupKeywords (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:241:7)
at /home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:233:38
at CodeGen.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:525:33)
at CodeGen.block (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:680:20)
at schemaKeywords (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:232:7)
at typeAndKeywords (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:161:3)
at subSchemaObjCode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:147:3)
at subschemaCode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:124:7)
at KeywordCxt.subschema (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:491:5)
at inlineRefSchema (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/vocabularies/core/ref.ts:40:26)
at Object.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/vocabularies/core/ref.ts:21:12)
at keywordCode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:523:9)
at /home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:228:21
at CodeGen.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:525:33)
at CodeGen.block (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:680:20)
at schemaKeywords (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:228:9)
at typeAndKeywords (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:161:3)
at /home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:100:5
at CodeGen.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:525:33)
at /home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:61:45
at CodeGen.code (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:525:33)
at CodeGen.func (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/codegen/index.ts:699:24)
at validateFunction (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:60:9)
at topSchemaObjCode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:94:3)
at Object.validateFunctionCode (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/validate/index.ts:42:7)
at Ajv.compileSchema (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/compile/index.ts:163:5)
at Ajv._compileSchemaEnv (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/core.ts:718:24)
at Ajv.compile (/home/fgeslin/simple-lb4-app/node_modules/ajv/lib/core.ts:370:34)
at createValidator (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/validation/request-body.validator.ts:235:18)
at validateValueAgainstSchema (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/validation/request-body.validator.ts:150:16)
at Object.validateRequestBody (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/validation/request-body.validator.ts:76:9)
at buildOperationArguments (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/parser.ts:92:9)
at Object.parseOperationArgs (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/parser.ts:48:10)
at processTicksAndRejections (internal/process/task_queues.js:95:5)
at /home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/providers/parse-params.provider.ts:68:22
at /home/fgeslin/simple-lb4-app/node_modules/@loopback/express/src/middleware-interceptor.ts:131:19
at /home/fgeslin/simple-lb4-app/node_modules/@loopback/express/src/middleware-interceptor.ts:131:19
at /home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/providers/send.provider.ts:46:24
at MySequence.handle (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/sequence.ts:291:5)
at HttpHandler._handleRequest (/home/fgeslin/simple-lb4-app/node_modules/@loopback/rest/src/http-handler.ts:115:5)
1) invokes POST /basics
`
## Expected Behavior
Ajv validation OK

## Link to reproduction sandbox

n/a

## Additional information

## Related Issues

n/a

_See [Reporting Issues](http://loopback.io/doc/en/contrib/Reporting-issues.html) for more tips on writing good issues_

Inizia da @loopback/rest/src/validation/request-body.validator.ts e segui la conversion dello schema del corpo della richiesta in AJV, usando il modello Basic e la riproduzione di POST /basics descritta qui. Controlla come vengono gestite le parole chiave example e examples dello schema OpenAPI, quindi verifica che la richiesta POST abbia esito positivo con la convalida AJV abilitata.

Scritto dal modello di indicizzazione a partire dal testo della issue.