What kind of change does this PR introduce?

This is a feature implementation

ClientSecretJWT and PrivateKeyJWT accept token_endpoint, claims, headers, and alg, but there is no way to control expires_in or issued_at for the generated JWT assertion. The underlying sign_jwt_bearer_assertion already supports these parameters, but they were not forwarded from the high-level classes.

Before:

 # No way to set a shorter token lifetime
 jwt_signer = ClientSecretJWT(token_endpoint="https://example.com/token")
 # Always generates a JWT with exp = iat + 3600 (1 hour)

Solution: Accept **kwargs in __init__(...) and forward them through sign() to client_secret_jwt_sign / private_key_jwt_sign, which already pass **kwargs to sign_jwt_bearer_assertion.

After:

 # Control expires_in and issued_at
 jwt_signer = ClientSecretJWT(
     token_endpoint="https://example.com/token",
     expires_in=300,  # 5-minute token lifetime
 )
 
 jwt_signer = PrivateKeyJWT(
     token_endpoint="https://example.com/token",
     issued_at=1700000000,
     expires_in=600,
 )

Fully backward compatible. No existing parameters changed. The kwargs only captures additional keyword arguments.

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover
• If this PR is about a new feature, or a behavior change, you have updated the documentation accordingly.

• You consent that the copyright of your pull request source code belongs to Authlib's author.