Add capabilities for listing procedures and functions in Postgresql and MySQL adapters

[matteoquintiliani]

I started developing to add the feature to list procedures and functions.

At the moment I have only developed for PostgreSQL and MySQL. It is necessary to extend the development for other types of DBMS.

This PR is required for developing list of stored procedures and functions in vim-dadbod-ui

This is the main issue: https://github.com/kristijanhusak/vim-dadbod-ui/issues/245