handle popular postgres extensions

[achristmascarl]

description

for types that aren't specifically handled, rainfrog just attempts to cast them to a string. for some of the most popular postgres extensions (starting with postgis and pgvector), we should make sure to explicitly handle those types.

proposed solution

explicitly decode types from popular postgres extensions

postgis

this might be a bit challenging, as sqlx doesnt support geometric types (launchbadge/sqlx#166). it looks like it may be possible to decode postgis types with georust (launchbadge/sqlx#166 (comment))

pgvector

there's pgvector-rust which might help