Integrate OpenBao chart

What does this MR do?

• Bump OpenBao chart to 0.3.1. https://gitlab.com/gitlab-org/cloud-native/charts/openbao/-/releases/v0.3.1
• Add developer docs with basic install instructions.
• Default OpenBao to the main rails DB.

What this MR does not do

• Automate auth setup per https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/openbao.md?ref_type=heads#configure-openbao-for-gdk.
• Automate enabling any OpenBao feature flags.

Related issues

Closes https://gitlab.com/gitlab-org/cloud-native/charts/openbao/-/issues/10

Author checklist

For general guidance, please follow our Contributing guide.

Test Plan

1. Install GitLab with openbao.install=true

   • Because of a known limitation this needs a existing certmanager.
   • Install it externally or install GitLab with openbao.install=false first and helm upgrade to openbao.install=true

2. Follow the instructions to initialize OpenBao

3. Confirm unseal secret is created and OpenBao Pods are healthy

   $ kubectl get pods -n glbao | grep openbao
   gl-openbao-768db4c678-lkdkp                    2/2     Running            0               3m42s
   gl-openbao-768db4c678-v94fp                    2/2     Running            2 (4m42s ago)   5m3s
   $ kubectl get secrets -n glbao | grep openbao
   gl-openbao-tls                    kubernetes.io/tls    2      31m
   openbao-unseal-keys               Opaque               5      5m2s

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart. - OpenBao for Operator is currently not supported.