feat: allow passthrough user to change password

[levkk]

• fix With PassthroughAuth set to enabled_plain, changing a user's password prevents new connections from being established #830
• fix [Auth] Passthrough auth for users without a password is broken #373
• fix: we were sending unnecessary Terminate message after FATAL error

[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 63 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/client/mod.rs	31.91%	32 Missing ⚠️
pgdog/src/backend/pool/connection/mod.rs	68.00%	8 Missing ⚠️
pgdog/src/net/messages/error_response.rs	40.00%	6 Missing ⚠️
pgdog/src/frontend/client/test/test_client.rs	93.15%	5 Missing ⚠️
pgdog/src/backend/databases.rs	96.58%	4 Missing ⚠️
...src/frontend/client/query_engine/two_pc/manager.rs	42.85%	4 Missing ⚠️
pgdog-config/src/general.rs	60.00%	2 Missing ⚠️
pgdog/src/backend/pool/cluster.rs	60.00%	2 Missing ⚠️

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