Allow `Avram::Credentials` to build unix sockets

If you need to build a unix socket, but want to use the Avram::Credentials object, it's a bit messy. I think we can make this easier to work with.

Here's an example of how to connect with a unix socket

# this ENV must be set otherwise it defaults to the current user's name on the system
# export PGDATABASE=my_db_development
Avram::Credentials.parse("postgresql:///my_db_development?host=/var/lib/postgresql/.s.PGSQL.5432&dbname=my_db_development")

I think you can also set a username and password using

Avram::Credentials.parse("postgresql://user:pass@/my_db_development?host=/var/lib/postgresql/.s.PGSQL.5432&dbname=my_db_development")

We would just need to make sure these methods account for the change

https://github.com/luckyframework/avram/blob/f5b000c26a5d04f54d166939cb1e7c1a3830da83/src/avram/migrator/runner.cr#L49-L59

Related: https://github.com/luckyframework/avram/issues/776

Start with src/avram/migrator/runner.cr lines 49-59 and inspect how Avram::Credentials.parse handles the PostgreSQL examples in the issue. Trace the methods used to build the connection and update the relevant behavior so credentials work with unix socket URLs. Done means the shown socket connection forms are supported, including optional username and password.