Make getFilterJsonSchemaFor() model-aware

@agnes512 ci sta già lavorando.

Dal 2/4/2020.

Currently, [`getFilterJsonSchemaFor()` isn't model-aware](https://github.com/strongloop/loopback-next/blob/85c12eb2ce12dfd54cc70d28d3d39e17abfe0142/packages/repository-json-schema/src/filter-json-schema.ts#L54-L55).

This means that the `openapi.json` order filter parameter will be generic:


In the case above, it's the `/todos` parameter from the `Todo` example.

This would lead to issues where the suggested `order` filter isn't a valid query:


(Notice the `"order": [ "string" ]`)

## Steps to reproduce

A quick way to reproduce is use the Todo example:

`lb4 example todo`

This also applies to any LoopBack 4 application that utilises `@param.filter(ModelHere)`.

## Expected Behavior

The `order` filter should be model-aware and not generic.

## Link to reproduction sandbox

`lb4 example todo`

## Additional information

```bash
$ node -e 'console.log(process.platform, process.arch, process.versions.node)' && \
> npm ls --prod --depth 0 | grep loopback
win32 x64 12.16.1
@loopback/example-todo@3.0.2 C:\Users\rifaa\Documents\loopback4-example-todo
+-- @loopback/boot@2.0.2
+-- @loopback/context@3.2.0
+-- UNMET PEER DEPENDENCY @loopback/core@2.2.0
+-- @loopback/openapi-v3@3.1.1
+-- UNMET PEER DEPENDENCY @loopback/repository@2.0.2
+-- @loopback/rest@3.1.0
+-- @loopback/rest-explorer@2.0.2
+-- @loopback/service-proxy@2.0.2
npm+-- loopback-connector-rest@3.6.0
ERR! peer dep missing: @loopback/core@^1.12.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/repository@^1.16.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/core@^1.12.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/repository@^1.16.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/core@^1.12.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/repository@^1.16.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/core@^1.12.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/core@^1.12.0, required by @loopback/model-api-builder@2.0.2
npm ERR! peer dep missing: @loopback/core@^1.12.0, required by @loopback/model-api-builder@2.0.2
```

## Related Issues

N/A

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