Support multiple DBs for loading/dumping of schema_migrations/

What does this MR do?

Make db/schema_migrations/ aware of multiple databases, and clean structure sql for for db/ci_structure.sql

In !64289 (merged), we introduced new db/ci_structure.sql for new ci database. But we disabled separate schema migrations, and cleaning because those helpers did not support multiple databases. This MR introduces that, and re-enables separate schema migrations, and cleaning for db/ci_structure.sql

Related issue: #334232 (closed)

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Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)