feat(apijson): move changes to new apijsoncustom package

Use `apijsoncustom` in `cloudflare_ruleset` only for now. This unblocks
bug fixes while we review the changes to be merged upstream.

Author: zakcutner

internal/apijson/decoder.go

@@ -31,10 +31,6 @@ type parsedStructTag struct {
 	// otherwise just use the UseStateForUnknown plan modifier
 	// NOTE #2: won't work if update behavior is 'patch'
 	encodeStateValueWhenPlanUnknown bool
-	// decodeZeroValueWhenNull indicates whether null and omitted values should
-	// be decoded as the zero value of the field type instead of leaving the
-	// field unset.
-	decodeZeroValueWhenNull bool
 }
 
 func parseJSONStructTag(field reflect.StructField) (tag parsedStructTag, ok bool) {
@@ -67,8 +63,6 @@ func parseJSONStructTag(field reflect.StructField) (tag parsedStructTag, ok bool
 			tag.noRefresh = true
 		case "encode_state_for_unknown":
 			tag.encodeStateValueWhenPlanUnknown = true
-		case "decode_null_to_zero":
-			tag.decodeZeroValueWhenNull = true
 		case "force_encode":
 			tag.forceEncode = true
 		}