Feature: define ignored PostgreSQL fields by using __ignore__ as source

[owahab]

One of the issues we came across was trying to create a SERIAL primary key in PostgreSQL that won't be copied and should not be inserted as NULL while copying from MongoDB.
This hack allows this behavior using a magical string __ignore__.

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1 out of 2 committers have signed the CLA.

✅ owahab
❌ rkhater
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