Allow configurable HTTP client for oauth2-client flows

[jgrandja]

We need to provide the capability for user's to configure the underlying HTTP client used in the various oauth2-client flows. For example, the user may need to set custom parameters and/or headers in the request to the /oauth2/token endpoint.

Providing a setRestOperations(restOperations) method in the classes that implement the specific oauth2-client flows should meet this requirement.

The tasks below compromise the oauth2-client flows:

• Allow configurable RestOperations for DefaultOAuth2UserService #5600 - Allow configurable RestOperations for DefaultOAuth2UserService
• Allow configurable RestOperations for CustomUserTypesOAuth2UserService #5602 - Allow configurable RestOperations for CustomUserTypesOAuth2UserService
• Add RestOperations implementation of OAuth2AccessTokenResponseClient #5547 - Add RestOperations implementation of OAuth2AccessTokenResponseClient
• Allow configurable RestOperations for NimbusJwtDecoderJwkSupport #5603 - Allow configurable RestOperations for NimbusJwtDecoderJwkSupport
• Register RestOperations @Bean to be used as default for oauth2-client flows #5607 - Register RestOperations @Bean to be used as default for oauth2-client flows