Describe `Git Access` in Pods architecture

What does this MR do and why?

This provides a first Git Access as a feature being impacted in Pods architecture.

This describes preliminary changes required, or evaluates the complexity of the proposal.

Additionally provides a template to use for other problems.

Rendered markdown: https://gitlab.com/gitlab-org/gitlab/-/blob/pods-blueprint-git-access/doc/architecture/blueprints/pods/pods-feature-git-access.md

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.