More robust testing strategy with real cassettes

[timothedelion]

• Add VCR cassette testing for realistic API testing without live calls in CI
• Create test suite for GitGuardianClient read-only methods (incidents, sources, honeytokens,
  occurrences, custom tags, tokens)
• Add secret redaction to prevent sensitive data in committed cassettes
• Add Makefile with test, test-vcr, test-unit shortcuts
• Split CI into separate unit and VCR test jobs
• I've not added the GITGUARDIAN_API_KEY on GitHub secrets manager and I assume the cassettes will all be recorded (and periodically re-recorded) from local.

Next steps :

• Also test "Write" API actions ?
• Use VCR instead of mocking also for tools tests (not only client tests)

[linear]

APPAI-157 Setup a better testing approach. Likely with VCR cassettes