[call creds] refactor token-fetching call creds to allow cleaner reuse (grpc#37510)

Previously, `grpc_oauth2_token_fetcher_credentials` provided functionality for on-demand token-fetching, but it was integrated into the oauth2 code, so it was not possible to use that same code for on-demand fetching of (e.g.) JWT tokens.  This PR splits that class into two parts:

1. A base `TokenFetcherCredentials` class that provides a framework for on-demand fetching of any arbitrary type of auth token.
2. An `Oauth2TokenFetcherCredentials` subclass that derives from `TokenFetcherCredentials` and provides handling for oauth2 tokens.

The `grpc_compute_engine_token_fetcher_credentials`, `StsTokenFetcherCredentials`, and `grpc_google_refresh_token_credentials` classes that previously derived from `grpc_oauth2_token_fetcher_credentials` now derive from `Oauth2TokenFetcherCredentials` instead, so there's not much change to those classes (other than a cleaner interface with the base class functionality).

The `ExternalAccountCredentials` class and its subclasses got more extensive changes here.  Previously, this class inheritted from `grpc_oauth2_token_fetcher_credentials` and fooled the base class into thinking that it directly fetched the oauth2 token, when in fact it actually performed a number of steps to gather data and then constructed a synthetic HTTP response to pass back to the base class.  I have changed this to instead derive directly from `TokenFetcherCredentials` to provide a much cleaner interface with the parent class.

In addition, I have changed `grpc_call_credentials` from `RefCounted<>` to `DualRefCounted<>` to provide a clean way to shut down any in-flight token fetch when the credentials are unreffed.

This PR paves the way for subsequent work that will allow implementing an on-demand JWT token fetcher call credential, as part of gRFC A83 (grpc/proposal#438).

Closes grpc#37510

COPYBARA_INTEGRATE_REVIEW=grpc#37510 from markdroth:token_fetcher_call_creds_refactor 3bd398a
PiperOrigin-RevId: 666547985

Author: markdroth