[typescript-fetch] Provide runtime checks that constraints are enforced

Description

When generating a typescritpt-fetch client library, the api constraints are not enforced. Servers may return values that are non-conformant but the client library will NOT fail fast.

Suppose that we have a swagger def like this:

  definitions: 
    StoreRating: 
      required: 
        - "store"
      properties: 
        store: 
          type: "string"
          description: "This is a store"
          example: "Storey Storis Store"
        rating: 
          type: "number"
          description: "How much we like the store"
          example: "0.881"
          minimum: 0
          maximum: 1
      type: "object"

The following is generated:

/**
 * 
 * @export
 * @interface StoreRating
 */
export interface StoreRating {
    /**
     * This is a store
     * @type {string}
     * @memberof StoreRating
     */
    store: string;
    /**
     * How much we like the store
     * @type {number}
     * @memberof StoreRating
     */
    rating?: number;
}

This does not do validation of the constraints that are imposed in the API. It should
do the following validations:

• verify that storeRating.store is not null
• verify that storeRating.rating is null OR a number between 0-1

Maybe this could be done by generating getters and setters, like this:

/**
 * 
 * @export
 * @interface StoreRating
 */
export class StoreRating {
    /**
     * This is a store
     * @type {string}
     * @memberof StoreRating
     */
    _store: string;

	get store(): string {
		return _store;
	}

	set store(store: string) {
		if (!store)
			throw new Error('Something bad happened');
		this._store = store
	}

    /**
     * How much we like the store
     * @type {number}
     * @memberof StoreRating
     */
    _rating?: number;

	get rating(): number {
		return _rating;
	}

	set rating(rating: number) {
		if (rating && (rating > 1 || rating < 0 )  )
			throw new Error('Something bad happened');
		this._rating = rating
	}

}

Swagger-codegen version

$ java -jar ../swagger-codegen-cli.jar version
2.3.1

Swagger declaration file content or url

---
  swagger: "2.0"
  basePath: "/api"
  paths: 
    /storeRating: 
      post: 
        responses: 
          200: 
            description: "Success"
            schema: 
              $ref: "#/definitions/StoreRating"
        operationId: "baz"
        parameters: 
        tags: 
          - "predict"
  info: 
    title: "Store Service"
    version: "1.0"
  produces: 
    - "application/json"
  consumes: 
    - "application/json"
  definitions: 
    StoreRating: 
      required: 
        - "store"
      properties: 
        store: 
          type: "string"
          description: "This is a store"
          example: "Storey Storis Store"
        rating: 
          type: "number"
          description: "How much we like the store"
          example: "0.881"
          minimum: 0
          maximum: 1
      type: "object"

  responses: 

Command line used for generation

java -jar ../swagger-codegen-cli.jar generate -i /tmp/baz.yaml -l typescript-fetch

Steps to reproduce

1. Run code generation

Run the supplied Swagger definition through the typescript-fetch generator first, then inspect the generated StoreRating model and the generator entry point used by the command. Done means generated clients enforce required, type, minimum, and maximum constraints at runtime, with the behavior verified against the reproduction.