Allow customization of `WebClient` used by `SpringReactiveOpaqueTokenIntrospector`

[pinguet62]

Expected Behavior

Allow custimization of WebClient used by SpringReactiveOpaqueTokenIntrospector.

Current Behavior

SpringReactiveOpaqueTokenIntrospector uses a basic WebClient:

WebClient.builder() /*setBasicAuth*/ .build()

Context

I have to add some behaviors to its WebClient: retry, metrics, ...

Previously I used the SpringReactiveOpaqueTokenIntrospector(String introspectionUri, WebClient webClient) constructor.
But now it's deprecated...

I think it can be done easily by adding a withWebClientBuilder(WebClient.Builder webClientBuilder) to its Builder.

Please let me know if it's a good idead!?

[jzheaux]

Closing in favor of #17196