feat(go): plugin implementation for Google Cloud SQL for PostgreSQL

[davidnastasi]

Added postgresql plugin

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[averikitsch]

Please update title to "feat(go): plugin implementation for Google Cloud SQL for PostgreSQL"

[averikitsch]

@pavelgj , please assign a Go reviewer. Thanks!

[apascal07]

I tried to resolve the conflicts in GH but I think you need to re-run go mod tidy.