Add support for reactive claim set converter in `NimbusReactiveJwtDecoder`

Expected Behavior

NimbusReactiveJwtDecoder should support converting the claim set in a reactive way.

Current Behavior

NimbusReactiveJwtDecoder only supports configuring a blocking/imperative claim set converter (added with #6080).

Context
We use a Spring Cloud Gateway that acts as an OAuth 2 resource server and validates JWTs issued by multiple different authorization servers.
One of these authorization servers issues tokens for which we need to pre-process the claims with information that is not statically available (one of the claims represents an ID that we need to look up in a different system first, and use the response to create additional claims).

Thus, we'd expect something like this in NimbusReactiveJwtDecoder:

public void setReactiveClaimSetConverter(Converter<Map<String, Object>, Mono<Map<String, Object>>> reactiveClaimSetConverter) {
	Assert.notNull(reactiveClaimSetConverter, "reactiveClaimSetConvertercannot be null");
	this.reactiveClaimSetConverter = reactiveClaimSetConverterSetConverter;
}

Of course the logic in https://github.com/spring-projects/spring-security/blob/53bc6a77963209bb2a5f3efdd855b1f4a8051cc3/oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/NimbusReactiveJwtDecoder.java#L182-L194 would have to be refactored to perform the conversion in a reactive way.

What do you think?

Start with oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/NimbusReactiveJwtDecoder.java, especially the claim conversion logic around lines 182-194. Trace how the existing blocking converter is configured and applied, then determine how reactive conversion should be exposed and integrated. Done means NimbusReactiveJwtDecoder supports the requested reactive claim set converter without removing existing imperative support.