Implement proper Rails authentication with OpenBao

In https://gitlab.com/gitlab-org/gitlab/-/issues/468922#note_1978126410, we decided to go with a simple use of the dev root token as we develop the secrets manager locally. But before we can use the features on production, we need to implement proper authentication with OpenBao.

We need to determine, do we want to use the OpenBao API proxy, or do we want to fetch the Secret ID through response wrapping like the example code in https://openbao.org/docs/agent-and-proxy/.