[REQ][Typescript] Add an option to generate an API Facade

Rationale

It's useful to have a single place to manage all logic relating to generated client stubs.
As it currently stands, the generated API stubs have to be manually imported and directly used.

Description

Sometimes, Configurations have to be computed during runtime, APIs may need proxying or rate limiting and so forth.
This also improves ease of maintenance- adding a new required configuration header, for example, could quickly become messy for larger projects. The same change would be made trivial with a facade.

Related issues/PRs

None found. Apologies if I've missed any.

Suggest a fix/enhancement

A very basic template could be used to generate a simple facade for all generated client API stubs.
Said template can then be left as is for a plug-and-play experience or tailored to the user's specific needs.

The generated facade could looks something like the below snippet.

If anyone else feels this may be useful, I can probably create a P.R for this.
      Cheers

Example facade.
Users can pass a static configuration object or a function to dynamically get the current configuration.
May be useful for some cases, mostly in stateful projects.

import { Configuration, AuthenticationApi, NotesApi, UsersApi } from '../api';

export type ConfigurationGetter = () => Configuration | undefined;

export class ApiFacade {

	private _config?: Configuration | ConfigurationGetter;

	/**
	 * API Docs go here
	 *
	 * @readonly
	 * @type {AuthenticationApi}
	 * @memberof ApiFacade
	 */
	public get authenticationApi(): AuthenticationApi {
		return new AuthenticationApi(this.getConfig())
	}

	/**
	 * API Docs go here
	 *
	 * @readonly
	 * @type {NotesApi}
	 * @memberof ApiFacade
	 */
	public get notesApi(): NotesApi {
		return new NotesApi(this.getConfig())
	}

	/**
	 * API Docs go here
	 *
	 * @readonly
	 * @type {UsersApi}
	 * @memberof ApiFacade
	 */
	public get usersApi(): UsersApi {
		return new UsersApi(this.getConfig())
	}

	public constructor(config?: Configuration | ConfigurationGetter) {
		this._config = typeof config === 'function' ? config() : config;
	}

	private getConfig(): Configuration | undefined {
		return typeof this._config === 'function' ? this._config() : this._config;
	}
}

Start by locating the TypeScript generator's templates and entry point for generated client API stubs. Compare the existing Configuration and API stub generation with the proposed ApiFacade example, then define how the option and generated output should behave. Done means the generator can optionally produce a facade that supports static or dynamic configuration without changing the default output.