Expose ID in API Event Entity

What does this MR do?

This Merge Request exposes Event IDs in API responses for external systems that are storing a log of the events to have a unique identifier for the events that are being submitted.

Additional details provided in #238141 (closed)

There is an additinal risks in this MR due to the updating of a new field that wasn't currently exposed via the Version 4 of the API. Given that this is an additive field, it shouldn't be introducing any additional integration risks.

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

No additional tests should be necessary given the existing tests suites should be validating the field is returned.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #238141 (closed)