[FEA] Unify API layers and remove the `mg` namespace

Is your feature request related to a problem? Please describe.
The unification of API layers and the removal of the mg namespace would simply the API and make it easier for new users to deploy cuVS in their projects.

Describe the solution you'd like
Instead of using the mg namespace, users would initialize a NCCL clique and set it on a resource handler. The user would make a generic call to the API and the resource handler would then inform cuVS on the appropriate algorithm implementation to run. For instance, if the resource handler contains a NCCL clique, it would run the MG implementation of the algorithm.

This change requires a modification of the entry functions called by the API. These would now be in charge of checking the presence of a NCCL clique on the resource handler and dispatch appropriately.

The issue names no files or tests. Start by locating the API entry functions and resource-handler types involved in NCCL clique handling, then trace how the MG implementation is selected. Done means generic API calls dispatch through the resource handler without requiring an mg namespace.