feat: maintenance mode per DB

[levkk]

fix #975

Allow the maintenance mode to be enabled/disabled on a per-database basis.

[codecov]

Codecov Report

❌ Patch coverage is 84.94208% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/client/mod.rs	13.33%	13 Missing ⚠️
pgdog/src/admin/maintenance_mode.rs	0.00%	11 Missing ⚠️
pgdog/src/backend/maintenance_mode.rs	96.61%	7 Missing ⚠️
pgdog/src/frontend/client/transaction_type.rs	33.33%	6 Missing ⚠️
...og/src/backend/replication/logical/orchestrator.rs	75.00%	2 Missing ⚠️

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