Extracted from #11084 (comment) Probably depends on #11084

For vault we may look into as becoming one of the supported auth methods
More details: https://developer.hashicorp.com/vault/docs/concepts/auth

I think our flow can work best similar to GitHub auth method. Where we pre-configure vault with a Coder OAuth app and then can login simply
With

vault login -method=coder token=$CODER_SESSION_TOKEN

Reference: https://developer.hashicorp.com/vault/docs/auth/github

Use case(s)

1. We can build a new module that automatically authenticates each coder workspace with vault without involving any 3rd party. We currently have a vault module that depends on GitHub auth.
2. Could also help realize User-level secrets #7087 by using Vault as the backend.