Add an option to ignore secrets which are removed by the scanned commit

Context

Add an option to ignore secrets which are removed by the scanned commit.
The goal of this PR is to create a way to accept secrets which are being remediated. If a dev is removing a secret from their code, but not rewriting the git history, GGShield will prevent them (pre-commit, pre-push or pre-receive) from committing and pushing the secret remediation.

This optional option will allow users to accept secrets in commits when they are removed, either on a deleted line or in a deleted file.

What has been done

Add --ignore-removed-secrets via the decorator add_secret_scan_common_options
Add an equivalent field to SecretConfig named ignore_removed_secrets

Validation

Create a repository, add then remove a secret:

git init
echo "ggtt-v-7faxcu9ak6" > secret.txt
git add secret.txt
git commit -m "add secret"
echo "remove" > secret.txt
git add secret.txt
git commit -m "remove secret"

Then verify that scanning the repository raises two incidents and only one with --ignore-removed-secrets

$ ggshield secret scan repo . --json | jq .total_incidents
Scanning... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 2 / 2
2

$ ggshield secret scan repo . --json --ignore-removed-secrets | jq .total_incidents
Scanning... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 2 / 2
1

PR check list

[x] As much as possible, the changes include tests (unit and/or functional)
[x] If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

GitGuardian / ggshield