add skip flag that can skip comparing source & destination schema when run splitdiff

[inolddays]

In splitClone state:
if source destination shard table has column like:
object_id varchar(128) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT ,
Then after copy and exec it on destination the table column will turn to like this:
object_id varchar(128) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT ,
In that case,(mysql's behavior) when source has too many tables contains columns as object_id.
there will be too much differ schema fail error out put on vtworkerclient side says:
remainder of the error is truncated because gRPC has a size limit on errors.
see the pic below:

This will obscure the real problem.
so add this flag assumed people already know the schema does not match and make the process going on

Signed-off-by: JohnnyThree whereshallyoube@gmail.com