feat(ci-server): add GitHub push event JSON parsing

[HotFazz]

Description

Add support for parsing GitHub push event JSON payloads so the CI-server can understand and extract relevant build information once webhooks are received

Expected outcome

The CI-server codebase contains a tested parsing component that can take a GitHub push event JSON payload and extract the branch name, commit information, repository URL, and pusher details without crashing.

Acceptance criteria

• A dedicated parsing class is implemented
• JSON parsing is implemented using Jackson
• The parser accepts JSON as input
• The following fields are extracted correctly:
  • ref --> branch name (strip refs/heads/)
  • after → commit SHA
  • repository.clone_url → repo URL
  • pusher.name → who pushed
  • head_commit.message → latest commit message
• Parsing function is unit-tested with a sample payload fixture
• Invalid/malformed payloads are handled gracefully (return error, don't crash)
• How to execute a push event successfully is documented in README.md
• Statement of contribution is added on the repository root

Related

GitHub push event payload