Split RESTUtils to links provider and injector

I'm building a Quarkus extension in which I would like to make a use of resteasy-links module. However, in order to make it easier I would like to make a few changes to the current implementation:

1. Added entityClassName field to the @LinkResource annotation. This will allow to generate JAX-RS resources with @LinkResource annotation during the build time in Quarkus.
2. Split RESTUtils class into two classes: ObjectLinksProvider and LinksInjector. This will allow to get links relevant to an object without injecting them into its field.
3. Add ClassLinksProvider. This will allow to get links relevant to a class (list and add operations that work with a given class). Currently RESTUtils only work with object instances.

Majority of the changes would be code reorganisation with a couple additions. This should not impact backwards compatibility.

I have raised a PR for this already: https://github.com/resteasy/Resteasy/pull/2311.