Brave Account endpoint client: introduce `Request<>` abstraction and unify HTTP method handling

[szilardszaloki]

Introduce a Request<> wrapper that unifies request bodies and HTTP methods, laying the groundwork for later attaching per-request network metadata (e.g. traffic annotations). Currently, APIRequestHelper uses a single net::NetworkTrafficAnnotationTag for all requests, which will be corrected in the migration to SimpleURLLoader.