Implement WithConnection for pgx and pgx5 database drivers

[bclarkx2]

Enable consumers to provide an existing *sql.DB with the option to close the connection without closing the underlying *sql.DB instance.

See #659 for a change doing the same for the lib/pq database driver, and #583 for MySQL.

Tests verified locally with:
make test-short DATABASE='pgx pgx5'

[coveralls]

coverage: 59.315% (+0.09%) from 59.226%
when pulling afde1f7 on nicheinc:master
into a5dc5d1 on golang-migrate:master.

[bclarkx2]

@dhui I updated this PR to handle the table-based locking introduced to the pgx driver in #992, so all the tests are passing now 🚀

Please let me know if there is anything else that could be done to improve this PR and move it toward the finish line. Cheers!

[bclarkx2]

@dhui Sorry to ping again, but I just wanted to check if there's anything else I can do to help move this PR forward. We're hoping to use this to implement a migrator that can be closed without closing the underlying connection pool using the pgx driver, so I'm very interested to help remove any blockers that remain!

Cheers