Add ability to return maps from rows after query

[ghivert]

Hi!

Currently, gleam_pgo will return all rows as tuples. To be able to reuse decoders between JSON and PGO, it would be nice to let the possibility for users to return rows directly.

An example:

pub type Dummy {
  Dummy(
    id: String,
    dummy: Int,
    field: String,
  )
}

pub fn encode(dummy: Dummy) {
  json.object([
    #("id", json.string(dummy.id)),
    #("dummy", json.int(dummy.int)),
    #("field", json.string(dummy.field)),
  ])
  |> json.to_string
}

pub fn decode(dyn: dynamic.Dynamic) {
  decode.decode3(
    Dummy,
    dynamic.field("id", dynamic.string),
    dynamic.field("dummy, dynamic.int),
    dynamic.field("field", dynamic.string),
  )(dyn)
}

pub fn query() {
  "SELECT id, dummy, field
   FROM dummies"
  |> pgo.execute(db, [], decode)
}