Prefix common class names with `Http`, apply `const` in a few more places, other cleanups.
Prefix common class names with `Http`, apply `const` in a few more places, other cleanups.

- Prefixing the class names makes it a little bit easier to work with them, since the
  short class names are otherwise quite generic. The `Http` prefix makes them a little
  easier to tie together/associate them with the `HttpClient` class at quick glance.
- A few methods were missing const on their arguments, probably because I overlooked
  it before.
- `HttpRequest::extra_requests_headers()` is renamed to `extra_headers()`, since
  the "request" part of the name was redundant. This also makes it more aligned with
  `HttpResponse::headers()` (which otherwise should be named `response_headers()`).

PiperOrigin-RevId: 409002707

google/federated-compute,google/federated-compute,google/federated-compute,google/federated-compute