Ability to add codecs via plugin system

[drewwestphal]

Feature description

It is currently not possible to add a custom scalar codec via the plugin system

Motivating example

Working on a project that has columns of type ltree and mutation function parameters of type ltree

#1772
https://discord.com/channels/489127045289476126/1157342633644003429

Supporting development

I [tick all that apply]:

• am interested in building this feature myself
• am interested in collaborating on building this feature
• am willing to help testing this feature before it's released
• am willing to write a test-driven test suite for this feature (before it exists)
• am a Graphile sponsor ❤️
• have an active support or consultancy contract with Graphile

[benjie]

Related: graphile/postgis#58

[jsmnbom]

I'm currently using a hacky Plugin like this (and excluding the native "PgCodecsPlugin". But having this built in would definitely be very helpful ^^
https://gist.github.com/jsmnbom/67c3be9c30b1df96ce652b9c23741c0e

[x] am interested in building this feature myself

tho I don't exactly know how it should look. Naively it is just implementing a hook inside getCodecFromType but, the t helper from the original source is quite useful and should maybe be exported in a more user friendly manner?

[benjie]

I'm getting back to V5 work again after a few months on other things (catching up, Worker, holidays, client work) and this is in my high priority list so leave it with me - I'll reach out if I can give some easy guidance.