Provide a SPI for the InjectorFactory

[gsmet]

In Quarkus, we end up having a copy of the following class just to override the InjectorFactory (with QuarkusInjectorFactory):
https://github.com/resteasy/resteasy-microprofile/blob/main/rest-client-base/src/main/java/org/jboss/resteasy/microprofile/client/RestClientBuilderImpl.java#L127

I wonder if we could expose a SPI for that so that Quarkus is not forced to have the RESTEasy CDI dependency.

Note that it's going to be a pain to migrate automatically to Jakarta but... at least, it will be done and will be more future proof.

[jamezp]

I'm thinking this is going to require using a ServiceLoader if that seems like an acceptable option. It looks like it's probably best to set the InjectorFactory early as it's used elsewhere in the ResteasyProviderFactoryImpl. I think this could be done either in RESTEasy or in RESTEasy MicroProfile. It would be somewhat of an enhancement to RESTEasy, however I could see where it's kind of a bug too.