Prune non-idempotent objects from pgx-generated SQL

[bitwalker]

I've reworked this PR from the ground up. It now depends on changes to pgx, which you can review here.

The changes here take advantage of the new #[to_sql] attribute I introduced, which provides us two options for customizing the SQL generator for specific entities:

1. We can disable SQL generation entirely for an entity, which I've done in this PR to the GapfillDeltaTransition type
2. We can handle generating the SQL ourselves, or redirect the generated SQL to an alternative output file. This is not demonstrated in this PR, though I've tested it; but we may want to consider that approach as a means for generating upgrades (there are a number of possibilities here, but the simplest would be generating a separate file for each non-idempotent entity, so that we can easily use those to check our handwritten SQL as well as write new migrations when things change).

As an unrelated aside: I noticed we have some extension_sql! calls still hanging around that do CREATE OPERATOR, we should remember to extract those to migrations.

[JamesGuthrie]

I like how concise this is with #[to_sql(false)]

[cevian]

LGTM