Simplify client credentials providers and deprecate RFC7523OAuthClientProvider

Add new simplified OAuth providers for machine-to-machine authentication:

- ClientCredentialsOAuthProvider: For client_credentials with client_id + client_secret
- PrivateKeyJWTOAuthProvider: For client_credentials with private_key_jwt (RFC 7523 Section 2.2)
- SignedJWTParameters: Helper class for SDK-signed JWT assertions
- static_assertion_provider(): Helper for pre-built JWTs from workload identity federation

The new providers set client_info directly in constructor, bypassing dynamic
client registration which isn't needed for pre-registered machine clients.

Deprecate RFC7523OAuthClientProvider: The original implementation incorrectly
used RFC 7523 Section 2.1 (jwt-bearer authorization grant) instead of the
intended Section 2.2 (private_key_jwt client authentication with
grant_type=client_credentials).

Also skip 3 flaky timing-dependent tests in test_stdio.py.

Author: pcarleton