Add support for multiple shard GTIDs in VGTID

VGTID is the offset that Vitess Connector receives from Vitess. Currently, one connector instance connects to one Vitess shard, and we need multiple connector instances to be able to connect to different Vitess shards. This is due to the initial connector design, which supports only one shard GTID in a VGTID.

We can improve it by add support for multiple shard GTIDs in a VGTID to the Vitess Connector. This way, we can have only one connector instance for multiple Vitess shards. It'll also facilitate adding support for the Vitess resharding operation since when resharding, the connector receives VGTIDs, each of which has multiple shard GTIDs.