Use RestClientBuilder, if available, to create RegistrationClient

[jacekbilski]

In SpringBootAdminClientAutoConfiguration there are two ways how RegistrationClient is created, either using existing WebClient.Builder bean or existing RestTemplateBuilder bean. My application has none, but it has RestClient.Builder bean configured and available. Please provide also a way to auto-configure Spring Boot Admin Client using RestClient.Builder.

For now, I had to create my own RestTemplateBuilder bean. Defaults are not good enough for me, because I need to authenticate each app using OIDC.

I might find the time to do it myself, maybe next week. In case I don't, I'm creating the issue here.