Implement PostgreSQL Metadata Provider #55

Job	Run time
Check & Build (macos-latest, stable)	10m 31s
Rustfmt	6s
Check & Build (ubuntu-latest-m, stable)	10m 50s
Check & Build (ubuntu-latest-m, stable)	13m 51s
Check & Build (macos-latest, stable)	11m 15s
Rustfmt	6s
	46m 39s

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