json marshal / unmarshal for hashid

[stereosteve]

Yesterday @schottra had mentioned using custom JSON marshal / unmarshal functions for playlist library... and it occurred to me we could experiment with that for hash ID encoding.

Might make sense to view the commits in series:

First one does developer apps which shows that for the vanilla case we don't have to define Full struct variants to override an ID column. In developer apps case, the min response is identical to full response, so we can also skip that step.

The second one uses trashid.TrashId for id field on users... while keeping user_id an int in the response (for your debugging pleasure!). This is a bit trickier as it gets into how sqlc.yaml field mapping works... but not too bad.

My main concern with this is that it is all up in the intersection of sqlc + golang + json in ways that might be confusing / surprising to devs. Like if we define a struct type in jsonb_types.go for a jsonb column... if you try to use trashid.TrashId in that struct it'll go badly because the JSON coming from the database will be a number but trashid.TrashId will expect to decode a hashid... and it'll fail.

We could probably define other type variants that just have MarshalJSON and not UnmarshalJSON. Or we could add some code to try to handle this case in UnmarshalJSON.

Maybe the move here is to expand this branch to use this custom type everywhere and see if it's overall good or bad.