Feature request: Add client_assertion field for oauth2 jwt-bearer grant type

The current OAuth2 configuration in the config package supports client_id and client_secret (either as strings or files).
Since PR #862, it also supports the Authorization Grant flow, where a private key can be used to sign a JWT.

It would be great if we could extend the OAuth2 struct to include client_assertion and client_assertion_file fields. This would enable users to manually provide a signed JWT when needed.

While the library can generate the signed JWT itself, there are cases where the client_assertion is created externally—such as by a sidecar, a specialized KMS, or a separate job—and then injected into the application as a file or environment variable. Currently, there's no built-in way to pass this parameter directly.

Start in the config package by locating the OAuth2 struct and the existing client_id and client_secret string-or-file handling. Trace how the Authorization Grant flow consumes these settings, then add coverage in the relevant config tests for directly supplied and file-based client assertions; done means both values can be provided and used for the JWT-bearer grant.