Add support for blocking clients

[Sineaggi]

Is your feature request related to a problem? Please describe.
We're using kotlin and spring mvc, not reactor and want to use a client with native webmvc support.

Describe the solution you'd like
With Spring 6.1 there's a new RestClient interface that's similar to WebClient and would provide support for blocking services running on virtual threads.
https://spring.io/blog/2023/07/13/new-in-spring-6-1-restclient

Describe alternatives you've considered
We're currently running with a forked client interface and implementation based off of the existing client code (with minor changes where web and restclients interfaces differ).

Additional context
Add any other context or screenshots about the feature request here.

[Sineaggi]

I've written up an initial implementation (mostly copy-pasted from the existing spring webclient implementation) master...Sineaggi:graphql-kotlin:restclient