Add parameter to drop fields

[niek-mereu]

Description of change

• Added a new parameter 'fields_to_drop' that enables us to drop fields from the mongodb source.
• Fixed the bug where the schema would be read from the row, even though it was found in meltano.yml. For both Incremental (key based) and full table replication.

QA steps

• automated tests passing
• manual qa steps passing (list below)

Risks

Rollback steps

• revert this branch

[cmerrick]

Hi @niekstrv, thanks for your contribution!

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