Add support for custom remediation messages

[fnareoh]

Context

This MR adds the display of custom remediation messages defined in the gitguardian plateform.

What has been done

This MR relies on the changes on pygitguardian GitGuardian/py-gitguardian#111

And for a custom remediation message to be display in ggshield the instance has to expose the messages through the metadata endpoint (see internal MR).

Validation

This version of ggshield should be compatible with versions of the app that do not yet expose the remediation messages (current staging for example) with the metadata endpoint and a version that does (review app of the MR that exposes to the public api).

Set the right instance for example using the env variable GITGUARDIAN_INSTANCE and GITGUARDIAN_API_KEY the try the ggshield secret scan pre-commit after leaking a secret and ggshield secret scan pre-push after commiting a secret but I am not sure how to test the pre-receive :(

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• 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.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.36%. Comparing base (ae78adc) to head (bc98683).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #928      +/-   ##
==========================================
+ Coverage   91.18%   91.36%   +0.17%     
==========================================
  Files         176      176              
  Lines        7252     7241      -11     
==========================================
+ Hits         6613     6616       +3     
+ Misses        639      625      -14     

Flag	Coverage Δ
unittests	91.36% <100.00%> (+0.17%)	⬆️

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[pierrelalanne]

Minor remarks, looks fine otherwise 👍