fix(mysql-cdc): fix auto schema mapping

[StrikeW]

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What's changed and what's your intention?

Previously we didn't init the upstream MySQL version to the SchemaDiscovery, so when it query the information_schema.statistics, it would not select the expression column.
The documentation and example of sea-schema doesn't reveal this details.

This PR reimplement the auto schema discovery of mysql-cdc, which only discover columns and would not discover indexes and foreign keys.

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[fuyufjh]

Such a pitfall! Thanks!