Decoupling collectives from UCX backend bootstrap

[sbahirnv]

Bootstrap of UCX Back-end implements necessary collectives for Realm Network Module and UCX back-end itself. That leads to every bootstrap option to implement these collectives. It leads to redundancy if we want to have different bootstrapping options but collectives from the same library. For instance, bootstrap the UCX backend using MPI or Point-2-Point but always use UCC collectives.

To address the issue, we want to decouple the collectives from the bootstrap and implement them in UCX Back-end itself. We are planning to have UCC collectives with options of multiple bootstrapping mechanism.