Fix #2: Qualify internal migrations table references

[tnht95]

Changes

• Schema-qualified all references to the internal migrations table to prevent them from being affected by changes to the session search_path.
• Added a configurable schema option (default: public) for the migration metadata table.
• Added a -s / --schema CLI flag to allow specifying the schema used for pgroll's internal migration table.
• Updated migration bookkeeping to execute sequentially after the migration succeeds, ensuring the migration state accurately reflects the database state.

Fixes #2

[scott-dn]

please add tests.