Include column types for composite type

[eaglesemanation]

With this addition, it should be possible to implement codegen on pgx/v5 side to resolve #2760
. Not sure for other drivers.

I intend to implement codegen as part of this PR as well, just want to get visibility as soon as possible.

[eaglesemanation]

Intentionally left endtoend test failing, until I verify that generated code does work as intended

[eaglesemanation]

Tested this, breaks if a column is an array of composite type. Otherwise works fine, except that pgx.Tx does not implement LoadTypes(...), will look into it soon

[PiotrBaczkowski96]

any update?

will it get merged it?

[eaglesemanation]

Doesn't look like anyone had a chance to review this PR, and I haven't reached out to anyone to speed this up. I'm surprised that it still doesn't have any conflicts, so I don't mind making small adjustments to get this merged if needed.

[andrewbenton]

I've been using a slightly modified version of this for a couple of months now. Overall, it's a really great improvement that I hope to see merged upstream soon.

I have noticed a few things that would help improve it:

1. toposort doesn't produce consistent ordering of composite types in generated files over multiple invocations of sqlc generate. I'm fairly certain that's due to the nature of Go's maps having a randomized key order over different process invocations. I think this could be corrected by making sortedCompositeTypes and toposort visit keys in alphabetical order.
2. Arrays inside of composite types don't seem to be supported. They'll be emitted instead as the non-array type of their value, e.g. bool array becomes bool, text array becomes text, etc
3. This could be my own misunderstanding or preference speaking, but I found that not changing the original DB interface was easier to work with. Instead, I introduced a new type TypeRegistrar interface that's able to do the type registration and emitted a func RegisterTypes(ctx context.Context, tr TypeRegistrar) error that can be used when setting up a connection. This allows types to be registered once right after the connection is established e.g. via pgxpool's AfterConnect hook.