Support mixed vendored and self-hosted models

What does this MR do and why?

Related issue: Support a mix of self-hosted and GitLab-hosted ... (#463917 - closed)

AI feature settings have the following UI/UX:

The idea is:

• When Self-hosted model option is chosen and configured, the features send requests to the self-managed AI Gateway with the params related to the self-hosted model
• When AI vendor option is chosen, the features send requests to the Cloud connector

It will allow non-air-gapped customers to either use self-hosted models for a feature or use the Gitlab vendor models

Implementation details

• Currently, AI Gateway URL can be configured either via AI_GATEWAY_URL env var or via Gitlab.config.cloud_connector.base_url
• This feature implies that customers use AI_GATEWAY_URL to configure their self-managed AI Gateway and Gitlab.config.cloud_connector.base_url to point to the cloud connector url
• When a feature is self-hosted, then AI_GATEWAY_URL value is used, when it's vendored then Gitlab.config.cloud_connector.base_url is used