[source-salesforce] enable changing cursor column

[LucasSegersFabro]

Case

I have a case where I sync some objects (Account) but there is a batch process that updates one column (and the default "SystemModstamp")
Thing is: not all the columns that get updated need to be fetched again for my batch

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Problem/Possible solution

On salesforce, I can simply configure another modstamp column that only gets updated if the set of columns I chose changes.
If I could use this custom column as a cursor, the syncs would pull MUCH less data and be really quick

BUT
The source-salesforce connector uses the SystemModstamp column as a "source defined crusor", making it impossible for me to configure another column as a cursor other than "SystemModStamp"

It would be nice if I could change the default cursor

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Some questions

If I were to attempt to contribute myself, I have some questions before understanding/digging into the airbyte source interfaces:

1. any pointers for "the cursor defines a default/suggested cursor, but make it not mandatory"?
2. if I "hack" into the actor definition and remove the source defined schema (or maybe set the column I need myself), would things fall apart?

It seems that the source-salesforce connector tries to infer the possible cursors