Use BuildMetadata to store build configuration in JSONB form

What does this MR do?

Yet another iteration of https://gitlab.com/gitlab-org/gitlab-ce/issues/50195

Yet another version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21464, this time using jsonb for PostgreSQL and text for MySQL.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides