aws-apigateway: JsonSchema expects type as enum type

### Describe the bug

When hand-writing a schema or importing a schema from somewhere else to use in multiple places, including apigateway, TypeScript rejects the schema:

```
const analyticsEventInputSchema = {
type: 'object',
properties: {
name: { type: 'string' }
}
}
const analyticsEventModel = api.addModel("AnalyticsEvent", {
modelName: "AnalyticsEvent",
contentType: "application/json",
schema: analyticsEventInputSchema,
});
```

### Expected Behavior

This should just work

### Current Behavior

> Types of property 'type' are incompatible.
> Type '"object"' is not assignable to type 'JsonSchemaType | JsonSchemaType[] | undefined'.

### Reproduction Steps

If the example above is not enough to show the issue, I can create a reproducing repo.

### Possible Solution

Introduce a string union type for `JsonSchemaTypeString` and use it here:
https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-apigateway/lib/json-schema.ts#L34

Keep the exported `JsonSchemaType` enum to avoid a breaking change and deprecate it.

### Additional Information/Context

_No response_

### CDK CLI Version

2.20.0

### Framework Version

_No response_

### Node.js Version

16.13.1

### OS

Windows 11

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start in packages/@aws-cdk/aws-apigateway/lib/json-schema.ts at the JsonSchemaType definition and reproduce the TypeScript example from the issue. Confirm that string-literal schema types are accepted while the exported enum remains available and is deprecated; add or update the relevant tests if the surrounding package identifies them.

Written by the indexing model from the issue text.