Improve `gitlab_schemas.yml` to include `gitlab_shared` and views

What does this MR do and why?

This updates gitlab_schemas.yml to describe:

• all data sources
• indicate shared models (like pg views or schema migrations)

Related to:

• !71540 (closed): parse all with pg_query
• #342378 (closed): Support database migrations via db/migrate
• #341783 (closed): #341783 (closed)
• !73480 (comment 721623517)

MR acceptance checklist

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