[OpenBao] GitLab HTTP Audit Log Connection

See context in #470461 (closed) -- after completing the design of the audit log streaming, we'll likely need a component in OpenBao to share audit logs with GitLab.

The OpenBao audit plugin code lives at https://github.com/openbao/openbao/tree/main/builtin/audit; any new plugin we create would be included there and referenced from the appropriate registry location: https://github.com/openbao/openbao/blob/b5cfc43f006f8c6fff4f6e7e1192fe4145a58c86/command/commands.go#L129-L134.

If it is sufficiently generic we can maintain it upstream otherwise we can build a custom plugin highly specialized to GitLab and maintain it in https://gitlab.com/gitlab-org/govern/secrets-management/openbao-internal or https://gitlab.com/gitlab-org/govern/secrets-management/openbao-mirror.