Add User-Agent to web hook service

What does this MR do?

Adds User-Agent to webhook service so that it doesn't get blocked by customers who filter out requests without a User Agent.

Closes: #18288 (closed)

Related to: #251227 (moved)

Use case

User-agent header not set in webhook calls -- because of our spam fighting measures, we require a user agent for all requests. Requests without user agents are summarily rejected.

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

@deuley can you review this or assign it to someone who can? 🙏