Support multiple client-registration-id configurations

[jkstrauss]

Multiple registration ids
In some environments it is required to support targeting multiple Oauth2 servers. The current solution for non-load-balanced feign clients only provide a single client-registration-id with the property spring.cloud.openfeign.oauth2.client-registration-id, so this is not possible.

Proposed solution
Add a new configuration: spring.cloud.openfeign.oauth2.client-registration-ids which should be a Map<String, String> mapping feign clients by name to a registration id. The default should still be spring.cloud.openfeign.oauth2.client-registration-id so there are no backward compatibility issues.

Other alternatives considered

• Using load-balanced clients
  • Downsides
    • A new spring.security.oauth2.client.registration and spring.security.oauth2.client.provider must be provided per each client if the clients use discrete names
    • Using the same name makes it difficult to do ad-hoc switching of hostname for a single client
    • The logs do not show the actual URL called, which makes debugging much more difficult
• Make a local custom OAuth2AccessTokenInterceptor