Add special handling for postgresql-specific migrations

[jvoisin]

This is a first step towards #1925

[jvoisin]

The linter complaining about singleCaseSwitch: should rewrite switch statement to if statement (gocritic) is idiotic, as this is the right construct for type checking.

[fguillot]

The linter complaining about singleCaseSwitch: should rewrite switch statement to if statement (gocritic) is idiotic, as this is the right construct for type checking.

What about adding a default statement in the switch block, and return an error saying that only the postgres driver is supported at the moment?