Implement instance level model selection in Duo Chat

We need to implement Instance level model selection for Duo chat.

How to prepare you dev environment

How to setup locally

See this comment.

Implementation proposal

See this MR. Find a way to make sure only have instance level model selection working when it the instance is SM in the same interface it is done in the MR.

Important files to start

Gitlab::Duo::Chat::ReactExecutor

Ai::ModelSelection::SelectionApplicable

Edited by Patrick Cyiza