[Rails] Add internal vs external OpenBao URL options

See second bullet point here: https://gitlab.com/gitlab-org/govern/secrets-management/deployment-investigation/-/issues/8#note_2482009632

OpenBao may have two separate URLs:

• Internal address used by Rails to talk to OpenBao. It may be cluster-local.
• External address used by Runners to talk to OpenBao. It will likely round-trip through the public internet.

The Rails configuration needs to be updated:

• Support both addresses.
• Use the internal address for all connections it initiates.
• Return the external address on Runner connection attempts.

This may warrant design doc updates to clarify that there may be an additional external LB between Runners and OpenBao, that is not present for Rails (e.g., when deployed on Kubernetes).

Related: #540873 (closed)