Increase max vrepl source def size from 64KiB to 16MiB (#10018)

This will allow us to avoid practical limits on the number of shard
and table definitions in workflows.

Signed-off-by: Matt Lord <mattalord@gmail.com>

go/vt/binlog/binlogplayer/binlog_player.go

@@ -560,7 +560,7 @@ func CreateVReplicationTable() []string {
 // AlterVReplicationTable adds new columns to vreplication table
 var AlterVReplicationTable = []string{
 	"ALTER TABLE _vt.vreplication ADD COLUMN db_name VARBINARY(255) NOT NULL",
-	"ALTER TABLE _vt.vreplication MODIFY source BLOB NOT NULL",
+	"ALTER TABLE _vt.vreplication MODIFY source MEDIUMBLOB NOT NULL",
 	"ALTER TABLE _vt.vreplication ADD KEY workflow_idx (workflow(64))",
 	"ALTER TABLE _vt.vreplication ADD COLUMN rows_copied BIGINT(20) NOT NULL DEFAULT 0",
 	"ALTER TABLE _vt.vreplication ADD COLUMN tags VARBINARY(1024) NOT NULL DEFAULT ''",