Create Endpoint to supply list of pre-created Flows to GitLab

Problem

As pointed out in gitlab-org/gitlab#559185 (comment 2732244754), the AI Catalog needs a way to understand which "GitLab" provided flows exist as well as their definition, both for allowing users to add and configure the flows for their project as well as using them as a template for creating their own flows.

Desired Outcome

Provide a way for the GitLab instance to request a list of GitLab provided flows, these should exclude the main chat flow.

Potential Solution

We already provide the GRPC contract for requesting a list of the available tools, we should be able to use the same way to provide a list of available flows. This should include the YML definition if this is available.