Add oauth2 token provider

[ozgen]

What

This PR adds OAuth2 client credentials support to gvm-auth.

It introduces a new OAuth2 token provider that can:

• request access tokens from an OAuth2 token endpoint
• cache tokens and refresh them when needed
• be used from both Rust and C code via a small C API
• use an injectable Clock abstraction to control time in unit tests
• provide CGreen-based C tests backed by a small Rust test server (gvm-auth-test-server) that simulates an OAuth2 token endpoint.

The implementation is covered by unit tests for successful token
retrieval, error handling, and token refresh behavior.

Why

Exporting reports to OpenVAS Intelligence requires OAuth2
authentication using client credentials.

By adding this support to gvm-auth, OAuth2 handling is implemented
once in a central place and can be reused by all components that need
to export data to OpenVAS Intelligence.

References

GEA-1531

Checklist

• Tests