feat: Implement GitHub and GitLab webhook handlers for pull/merge req…

[rostilos]

…uest and branch events

• Added GitHubPullRequestWebhookHandler to handle pull request events from GitHub.
• Introduced GitHubWebhookParser for parsing GitHub webhook payloads.
• Created GitLabBranchWebhookHandler for handling branch push events in GitLab.
• Developed GitLabMergeRequestWebhookHandler to manage merge request events from GitLab.
• Implemented GitLabWebhookParser for parsing GitLab webhook payloads.
• Refactored AllowedCommandUserService and ProjectService to utilize unified VCS repo bindings.
• Updated ReviewRequestDto to include GitLab as a supported VCS provider.

[coderabbitai]

Important

Review skipped

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Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

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