Extend postgres compatibility to enable golang migrate

[sleepycat]

What would you like to be added or enhanced

Golang migrate is a popular library for running migrations against a variety of databases, including postgres.
Despite recent work on improving postgres compatibility, attempting to use the migrate tool with immudb currently fails with the following error:

$ migrate -database "postgres://immudb:immudb@localhost:5432/defaultdb?sslmode=disable" -path migrations up
error: failed to open database: pq: illegal arguments: unknown function current_schema in line 0: SELECT CURRENT_SCHEMA()

Why is this needed

Targetting the postgres features needed for the migrate tool would enable immudb users to use migrate to manage schema evolution.