js_client: add optional request options parameter

[AndreyVMarkelov]

Summary

This change adds an opt-in --request-options flag to the JavaScript client backend.

When enabled, generated client methods:

• accept an optional options parameter;
• document it in JSDoc;
• forward it as the final argument to this.request(...).

For example:

routes.filesUpload = function (arg, options) {
  return this.request(
    'files/upload',
    arg,
    'user',
    'content',
    'upload',
    'files.content.write',
    options
  );
};

The generated output is unchanged unless --request-options is explicitly enabled.

Motivation

Some SDKs need per-request transport options that are separate from API request arguments, such as request cancellation (AbortSignal), request timeouts, or future transport-specific behavior.

This change provides a generic mechanism for JavaScript client generators to expose those options without changing generated API request types.