Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
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Proposed Feature

I'd like to see a health check on the external webhooks receiver, probes that verify it, on a known URL so I can hook it up to my load balancer.

Motivation

This would improve reliability of the external webhooks receiver by making it respond to health checks in general.