Very WIP: Custom Postgres Types in Macros

[phated]

As @abonander mentioned in #313, his ab/offline branch has laid the groundwork for working on custom type support in postgres.

I know this draft has my custom types and their hardcoded oids, but I really need custom type support in macros, so I've been hacking around in the project a bit.

If I'm on the right track to adding full support for custom types, I'm happy to keep trying to implement them with some guidance.

If I'm completely off-track, we can close this and I will build against my branch until the feature gets finished.

Keep up the great work! 🍻

[mehcode]

First I want to say that I didn't mean to close this without a note. Apolgoies.

After skimming your PR, you have the basic idea. A discussion point is how does the user add stuff to "get-custom-types"? We're currently leaning towards a sqlx.toml file where you declare type mappings
of "SQL type name" to "full Rust type path".

If you get around to rebasing this for personal usage or to try and move along this task on SQLx, note that you want to declare your custom types with PgTypeInfo::with_name("dfsdsf") or PgTypeInfo(PgType::DeclareWithName("dsfsdf")). This will cause SQLx to resolve the OID at runtime (and cache it).

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Unfortunately this would only help Postgres (as other databases have column local types not types as first class objects) so we've placed this after #339 in order of priority.