Move provider credentials under ProviderConfig

[eleftherias]

Currently we have our GitHub provider credentials defined in the server-config.yaml like this:

github:
  client_id: X
  client_secret: Y

We fetch the credentials using viper directly: https://github.com/stacklok/minder/blob/4524f991586d2b90603718364eef0a0068e85219/internal/auth/oauth.go#L121-L122

Since we now have the ProviderConfig, we should move the credentials there:

provider:
  github:
    client_id: X
    client_secret: Y

and refactor the oauth client to use the server configuration, instead of calling viper directly.