Adds pgt_outbox_setup and pgt_outbox_events for Transactional Outbox Pattern

[bougyman]

This adds two new methods, which implement the transactional
outbox pattern utilizing postgresql triggers.

This only handles creating the outbox table, and writing events
to the outbox for table changes, no other part of the system.

Context:

• https://event-driven.io/en/outbox_inbox_patterns_and_delivery_guarantees_explained/
• https://gitlab.com/os85/tobox#usage

[jeremyevans]

Thank you for working on this. There seems to be a significant amount of inherent complexity (17 options!). I fear it may be too specialized to your needs, even though I can see you've spent significant time trying to make it more generalized (hence the large number of options). I'm also not sure usage of this feature would be high enough to justify including it. However, I could see it saving someone a lot of time if this meets their needs. So I'm on the fence about whether to merge it.

In terms of implementation, testing looks good, modulo unnecessary whitespace changes. Forcing timestamptz is probably not desirable. It's better to use Time, and have users that want timestamptz instead of timestamp use the pg_timestamptz extension.

[bougyman]

In terms of implementation, testing looks good, modulo unnecessary whitespace changes. Forcing timestamptz is probably not desirable. It's better to use Time, and have users that want timestamptz instead of timestamp use the pg_timestamptz extension.

Made the switch to Time here, thank you for that suggestion, I was unaware of pg_timestamptz.

In terms of options, I wanted to support the various 'outbox/inbox' frameworks I've seen (like tobox, but not limited to it), and this set of 17 options (10 of these are simply column names) with the two conditionals supports almost all that I've seen during my research the last few weeks. And of course it acts very sane and normal in the Basic setup passing only the name of the table to fire outbox events for. I hope to see it included as we already leverage this gem for other triggery behaviors throughout our architecture.

modulo unnecessary whitespace changes

Put the whitespace back like it was :)

[bougyman]

There wasn't much shared with the rest of this library, so I'm going to publish this as its own gem instead. Thanks for the consideration. https://github.com/rubyists/sequel-pgt_outbox

[jeremyevans]

Sounds good. Might be useful to link to your gem from Sequel's plugins/extensions page (https://sequel.jeremyevans.net/plugins.html), so send a PR to the Sequel repository if you would like to do that.