Add OpenBao proxy to GDK

We currently can run OpenBao server with GDK: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/openbao.md?ref_type=heads

But this won't be enough for building the secrets manager features because we also need to run the OpenBao proxy.

For sample proxy configuration we can follow !162431 (merged).