CID-2777: Secure webhook listener endpoint

[mohamedlajmileanix]

🛠 Changes made

This PR adds security measurements to only accept trusted webhook events.
All events include X-GitHub-Enterprise-Host header, indicating the host of GHE, which should be the same as the ghe_url passed as docker command argument.
GitHub also offers another layer of securing the events, by adding a signature header to the event. This signature is based on a secret, set by the customer. If the customer wishes to use this option, they need to also add this secret as docker command argument, to be provided to the agent to validate the events' signature.

✨ Type of change

• New feature (non-breaking change which adds functionality)

🧪 How Has This Been Tested?

• GitHubWebhookControllerTest for different responses
• GitHubWebhookHandlerTest for validating headers

🏎 Checklist:

• My code follows the style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (README.md)
• My commit message clearly reflects the changes made
• Assigned the appropriate labels (version, PR type, etc.)

[github-actions]

LeanIX GitHub Agent Code Coverage

File	Coverage [80.17%]	🍏
GitHubWebhookHandler.kt	100%	🍏
GitHubWebhookController.kt	100%	🍏
GitHubEnterpriseProperties.kt	93.75%	🍏
WebhookEventService.kt	83.97%	🍏
GitHubWebHookEventHelper.kt	79.71%	🍏
GlobalExceptionHandler.kt	51.92%	❌
Exceptions.kt	38.24%	❌

Total Project Coverage	67.79%	🍏