Enable PGMQ to run without installing an extension

[axelfontaine]

This pull request enables the installation of PGMQ using a simple psql command:

psql < pgmq.sql

It does NOT affect the existing functionality, nor does it impact PGMQ's ability to be installed as an extension.

This addresses the following issues directly and inderectly:

• Question: Does this work on hosted providers? #205
• Don't assume to be installed as an extension #347
• Add pgmq extension to Microsoft Azure Postgres flexible server (upvote link) #373
• GCP Support #378

To achieve this the following was changed:

• The pgmq schema gets created if it doesn't already exist
• The actual implementation of send and send_batch are moved higher up the SQL file so they can be found when referenced by the convenience functions with less arguments
• A new _extension_exists function is introduced to check the existance of arbitrary extensions
• The implementation of _ensure_pg_partman_installed has been retrofitted to use the new _extension_exists function.
• All references to the pgmq extension have been guarded with _extension_exists('pgmq')

[ChuckHend]

This is awesome, thank you for doing this @axelfontaine. I will look it over in the next few days.

[ChuckHend]

image builds are going to fail for PR coming from an external repo (we still need a better way to configure that in CI), so we can ignore those failures.

[v0idpwn]

This is great. I think we should have tests run on it, though.