Model Switching - Request to the AIGW with top namespace preferences

(The current issue is subject to change) Model Swithcing when users uses AI feature through a namespace we need to signal to the AIGW what model needs to used according to the admins preferences.

Just like for Self-hosted models, send the models metadata but the just the provider 'gitlab' (working name needs to be refined) and the model identifier as shown in the example below:

model_params: { 
  'provider': 'gitlab',
  'identifier': 'claude_3_7_sonnet'
}

Make sure the code behind the ai_model_switching feature flag.

Default models and fallback mechanism

In order to ensures a structured decision-making process and gives users autonomy without breaking organizational policies follow the default models and fallback mechanism as describe below.

1. check current namespace top-parent preferences for model assigment for the feature.
2. If no top-level group assigments, keep default default behavior
3. If the top-level group assigments has blank offered_model_ref, keep default default behavior