Add sarif support

[montyly]

This PR contains the changes made by @bernard-wagner in #903, plus a couple of minor improvements

The steps to use Sarif are the same as in #903:

$ slither [path to target] --sarif slither.sarif
$ curl -u $USER:$TOKEN -X POST -H "Accept: application/vnd.github.v3+json"  https://api.github.com/repos/$ORG/$REPO/code-scanning/sarifs  -d '{"commit_sha":"$COMMIT","ref":"refs/heads/$BRANCH","sarif":"'$(gzip -c slither.sarif | base64 -w0)'"}'

In the long term, we probably want to move the sarif conversion to the Output class; but for now I will keep it as it is