Add a `score` field to `ScannerResult`

[willdurand]

We discussed that with @diox in Slack. We'd like to extract the different score values returned by the ML API and store them in the scanner results.

We'll have an aggregate score that we'll set on the ScannerResult created by the call_ml_api task (see #7209).

We'll have one score per scanner result sent to the API and we'll set this score to the corresponding ScannerResult (customs results will always have a score, for other scanners we'll see).

This issue is related to #7210.

[willdurand]

I requested a change in the API contract of the mad server (because this contract has changed). Once we have that, we'll be able to extract and store the score easily.

[AlexandraMoga]

There is a new 'Score' column in scanner results which is populated with - (yara and wat) or n/a (customs). By reading the conversation from the pull request, I get it that these values might change in the future.

@willdurand Is there an interpretation to this score right now or was this issue just about adding the new column?