Allow specifying optional provider credentials URL in `ProviderMetadata`

A very trivial enhancement. Providing a URL where to get credentials is crucial for users to know where to go if they have to manually configure the provider.

Case in point: https://github.com/WordPress/wp-ai-client/blob/trunk/includes/API_Credentials/API_Credentials_Settings_Screen.php#L121

This is optional, since not every provider will require this (although today's cloud providers all do).