NUMERIC reads: full digits arrive, the decode drops them #826

The read-side follow-up promised in crate/sqlalchemy-cratedb#300, with the measurement #652 was missing. As @matriv said there, the full number reaches the client; the digits are dropped by this driver's decode.

On CrateDB 6.4.2, with 1.234567890123456789012345 stored in a NUMERIC(38, 24) column, the raw /_sql response carries every digit:

{"cols":["n"],"rows":[[1.234567890123456789012345]],"rowcount":1,"duration":105.078}

cursor.fetchone()[0] on the same query returns 1.2345678901234567, a float. The response is decoded with orjson.loads (http.py line 244), which parses every JSON number to a float64 and has no parse_float hook; stdlib json.loads(raw, parse_float=Decimal) on those bytes returns Decimal('1.234567890123456789012345').

Writes are already exact, since Decimal goes out as a string (#751). A stdlib decode with parse_float=Decimal would make reads match, but it changes the returned type for every float column and gives up orjson's speed, so it likely wants to be a connection option rather than the default.