Mmillet/sca 1670 refactor ggshield scan ci commands

Context

Refactor sca scan ci and iac scan ci: we want to

compute the range of commits associated with the merge request, and not the push event
only allow the command to run in merge requests pipelines (for now, a fallback behaviour is kept, so that the command will perform a full scan if that is not the case).

What has been done

Refactor the commands

Validation

Run the commands in merge request pipelines, with verbose mode.

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.

Codecov Report

Attention: Patch coverage is 69.41176% with 26 lines in your changes missing coverage. Please review.

Project coverage is 91.32%. Comparing base (372be8a) to head (cd9c929). Report is 69 commits behind head on main.

Files	Patch %	Lines
ggshield/cmd/iac/scan/ci.py	14.28%	12 Missing :warning:
...shield/core/git_hooks/ci/get_scan_ci_parameters.py	75.51%	12 Missing :warning:
ggshield/cmd/sca/scan/ci.py	86.66%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #902 +/- ## ========================================== - Coverage 91.67% 91.32% -0.35% ========================================== Files 170 173 +3 Lines 7060 7181 +121 ========================================== + Hits 6472 6558 +86 - Misses 588 623 +35 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/902/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/902/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.32% <69.41%> (-0.35%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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