Use mutex in OIDC configuration handlers

[austingebauer]

This PR adds use of the oidcLock in configuration handlers for Vault OIDC provider. The purpose of using this lock is to avoid race conditions where concurrent configuration changes could leave relationships between resources in an inconsistent state. For example, we implement checks to ensure that named resource references (e.g., client->assignment, provider->scopes_supported) exist. Using a mutex for writes ensures that the reference checks won't race.

The mutex is not used to protect unsafe memory in this case. All in-memory data structures (go-memdb, go-cache) internally use a mutex or provide safe concurrent access through other means.