AI Catalog: Flow editor - Select a single agent

Problem to solve

We want to build a node-based editor for flows and we're covering that in #553913 (closed) but that work is more involved and will take some time. In the mean-time, we need a simple UI that allows us to create and edit flows, just without all the bells and whistles of the node-based option.

Proposal

Add a single agent selector field that can be expanded in the future to allow for multiple nodes.

Here the concept design for the multi node builder:

Start small, keep it simple, and iterate.

More Context

• @leetickett-gitlab has started an MR to associate agents with flows. The mutation arguments will probably evolve in iterations as we come to support more functionality around this
• Related Backend Issue