feat(auth): add arbitrary list methods metadata field

[GeorgeMac]

This adds a space for (relatively) arbitrary metadata for each authentication method.
Ultimately, this will support the UI is presenting appropriate login features.

e.g. Login with Google button.

There is a map[string]string metadata field on the auth method info message payload.
This can be configured by adding metadata in internal/config on the call to Info().

I have added an initial example where the OIDC method adds all of its providers authorize and callback URLs.
The UI will be able to use this information to correctly configured a login button based on what is actually configured in the backing Flipt instance.

For example:

{
  "methods": [
    {
      "method": "METHOD_TOKEN",
      "enabled": false,
      "sessionCompatible": false,
      "metadata": {}
    },
    {
      "method": "METHOD_OIDC",
      "enabled": false,
      "sessionCompatible": true,
      "metadata": {
        "provider.google.authorize_url": "/auth/v1/method/oidc/google/authorize",
        "provider.google.callback_url": "/auth/v1/method/oidc/google/callback"
      }
    }
  ]
}

[markphelps]

🔥