Support for GCP Vertex AI and/or Amazon Bedrock

What feature would you like to see?

Users should be able to use the Codex App using GCP Vertex AI or AWS Bedrock instead of OpenAI directly.

Additional information

This is important for my workplace to try Codex; we have to go through a major cloud provider for compliance reasons, in fact we have some projects which can _only_ use AWS Bedrock. (Claude Code supports Vertex and Bedrock so we use that quite heavily.)