[Task]: Implement async scanners

[willdurand]

Description

The idea is to allow scanners to be called asynchronously. In my mind, that means a scanner marked as "asynchronous" is called with a scanner_result_id and we don't store any results (we only expect the scanner to return a 20x response). Later, the scanner will use this id to send a PATCH request with its results. That's when we'll store the results.

Acceptance Criteria

see above

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