Add OAuth + RestTemplate support

[rwinch]

Summary

We should provide a simple way to integrate OAuth support with RestTemplate. This support would be similar to the WebClient support, but for RestTemplate.

The WebClient extension for Servlet-based applications is ServletOAuth2AuthorizedClientExchangeFilterFunction.

See the sample oauth2webclient and how to configure and use it.

There are a few tasks involved in order to reach feature parity between ServletOAuth2AuthorizedClientExchangeFilterFunction and RestTemplate support. Here is a break-down of the tasks involved.

• User should be able to set the current client using either an OAuth2AuthorizedClient or clientRegistrationId
• Each request should contain the Authorization header with Bearer <access-token> for the current OAuth2AuthorizedClient
• Get a new access token if it's not available for the current clientRegistrationId
• Refresh an expired access token - applicable to authorization_code grant clients
• Get a new access token when current token expired - applicable to client_credentials grant clients
• Add clock skew support for access token expiry time

[jgrandja]

Partial support for RestTemplate has been added in 5.1.
See Token Request/Response and UserInfo Request/Response.