Fix simple protocol encoding of json.RawMessage

The underlying type of json.RawMessage is a []byte so to avoid it being
considered binary data we need to handle it specifically. This is done
by registerDefaultPgTypeVariants. In addition, handle json.RawMessage in
the JSONCodec PlanEncode to avoid it being mutated by json.Marshal.

https://github.com/jackc/pgx/issues/1763

pgtype/pgtype_default.go

@@ -1,6 +1,7 @@
 package pgtype
 
 import (
+	"encoding/json"
 	"net"
 	"net/netip"
 	"reflect"
@@ -173,6 +174,7 @@ func initDefaultMap() {
 	registerDefaultPgTypeVariants[time.Time](defaultMap, "timestamptz")
 	registerDefaultPgTypeVariants[time.Duration](defaultMap, "interval")
 	registerDefaultPgTypeVariants[string](defaultMap, "text")
+	registerDefaultPgTypeVariants[json.RawMessage](defaultMap, "json")
 	registerDefaultPgTypeVariants[[]byte](defaultMap, "bytea")
 
 	registerDefaultPgTypeVariants[net.IP](defaultMap, "inet")