Add MongoDB processor find-all operation

[webfrank]

Hi, I've added find-all operation to MongoDB processor. Cannot use bson.MarshalExtJSON because for an array it returns the Error: WriteArray can only write a Array while positioned on a Element or Value but is positioned on a TopLevel

[Jeffail]

Looks like we'll need to work around the array issue by iterating the returned BSON objects into a []json.RawMessage and then marshal that into the final binary format. We also ideally need at least one unit test. I can do those two on my side when I merge but you need to add the sign off: https://github.com/benthosdev/benthos/pull/1969/checks?check_run_id=14410191847

[webfrank]

Hi, don't know if I've done it correctly.

[Jeffail]

Running git commit --amend -s would've done it, it just requires Signed-off-by: foo <bar>, looks like the PR got blown up from a merge (reporting 41 files changed).

[CLAassistant]

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1 out of 3 committers have signed the CLA.

:white_check_mark: Jeffail
:x: Francesco Boscarino
:x: rasnes

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