Enforce support for connection variables in MySQL Bulkload actions, Snowflake and CombinationLookup

[xdelox]

Ideally fixes #4054, #4055 , #4056, #3833

I said "ideally" because most of things had already been done, meaning that the code looked already migrated, apart small details in dialogs.

• With regards of MySQL Bulk load actions (to and from file), [Feature Request]: MySQL Bulk Load Action: manage connection through variables #4055 and [Feature Request]: MySQL Bulk Load from File Action: manage connection through variables #4056, I've added an Integration Test, but I've disabled it as it requires the MySQL jdbc connector (not provided out-of-the-box).
• Concerning [Feature Request]: CombinationLookup - specify connection through variables #3833, it was ok, there is already an IT using variables among the database project, PostgreSQL is the target DB
• Last, but not least, also Snowflake looked migrated, but I had no chance to test it. Could anyone provided with a working account test it, please?

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