Typescript-Angular: Fix several query parameters serialization issues

[VladimirSvoboda]

This notably fixes:

• JSON query param serialization
• array serialization with style=form and explode = true

The first commits of the patch introduces integration tests for query
parameters serialization in
samples/client/others/typescript-angular-v20:

• samples/client/others/typescript-angular-v20/projects/app/src/api.query_param_deep_object.spec.ts (move of an existing test)
• samples/client/others/typescript-angular-v20/projects/app/src/api.query_param_form.spec.ts (new test)
• samples/client/others/typescript-angular-v20/projects/app/src/api.query_param_json.spec.ts (new test)

They should be executed as part of CircleCI.
The last commit fixes the issue.
Fixes are in the following files:

• modules/openapi-generator/src/main/resources/typescript-angular/api.base.service.mustache
• modules/openapi-generator/src/main/resources/typescript-angular/api.service.mustache
• modules/openapi-generator/src/main/resources/typescript-angular/encoder.mustache

As the class HttpParams from Angular is specially designed for the
mimetype: application/x-www-form-urlencoded it does not support
the range of query parameters defined by the OpenAPI specification.
To workaround this issue, this patch introduces a custom
OpenAPIHttpParams class which supports a wider range of query param
styles.

Note that as HttpClient is used afterwards, the class
OpenApiHttpParams has a method to convert it into a HttpParams from
Angular with a no-op HttpParameterCodec to avoid double serialization of
the query parameters.

Fixes #21934.

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