Disable REST codegen if resteasy is not present

A user reports a conflict https://github.com/quarkusio/quarkus/issues/19868
when resteasy reactive is being used. It is not possible to mix and match as we don't support resteasy reactive; moreover, we force the resteasy dependency on end users.

We should not bring in the dependency unless the capability is available.
the quarkus extension should check if the resteasy capability is present and disable REST codegen otherwise.

the new programmatic API won't be affected, as users will write the endpoints themselves.

Additionally, resteasy reactive should be compatible with plain resteasy, so verify detecting the capability for reactive may allow to enable codegen too