feat(pg:upgrade): support essential dbs

[binarycleric]

This code is still a work-in-progress but I am just trying to get an idea of what changes would be required in the CLI for us to support pg:upgrade on the essential tier.

Testing

These command tests check the warning output of the command based on the type of db's included in the upgrade. Once you've seen the message, Cntrl + C to cancel the upgrade.

This tests a follower db

• ./bin/run pg:upgrade HEROKU_POSTGRESQL_BRONZE -a heroku-cli-test-staging -v 14
• Check that the warning message includes the followed db --> (HEROKU_POSTGRESQL_CRIMSON)
• Cntrl + C to cancel the upgrade.

This tests a non-follower db

• ./bin/run pg:upgrade HEROKU_POSTGRESQL_TEAL -a heroku-cli-test-staging -v 14
• Check that the warning message does not include any followed dbs like so...

postgresql-objective-63454 will be upgraded to a newer PostgreSQL version.

  This cannot be undone.

• Cntrl + C to cancel the upgrade.

[k80bowman]

I had a couple of questions on this one.

[k80bowman]

This works and looks good. Thank you!