Add cleartext password authentication

[SeanTAllen]

Summary

• Add support for PostgreSQL cleartext password authentication (auth type 3)
• Previously, servers requesting cleartext auth would get pg_session_authentication_failed with UnsupportedAuthenticationMethod — now the driver sends the password and completes authentication normally
• No public API changes: the driver detects the server's requested method automatically using the existing DatabaseConnectInfo password

Test plan

• New parser unit test: ResponseParser/AuthenticationCleartextPasswordMessage
• New parser chain test: ResponseParser/MultipleMessages/CleartextFirst
• New mock server auth success test: Cleartext/AuthenticationSuccess
• New mock server auth failure test: Cleartext/AuthenticationFailure
• Updated PBT generator to include cleartext auth messages
• Updated existing tests that used type 3 as "unsupported" to use type 2 (KerberosV5)
• Counterfactual checks on all 4 new tests
• Full unit test suite passes (417 tests)