Broken out of #4150

Add a generic webhook receiver that can use user-defined expressions to parse arbitrary JSON request payloads to identify the request's repository of origin. This information can be used to refresh a Warehouse, prompting an artifact discovery cycle.

Such a receiver will be useful for integrating Kargo with lesser-known platforms for which we haven't/won't build specific receivers. It will also be useful for cases such as ECR or GAR, which do not natively support notifying external systems of pushed artifacts via webhooks. Users can jury rig something that accomplishes this, but (to the best of my knowledge) there is no official, or even de facto, standard for such payloads, meaning Kargo will require the flexibility to parse whatever users elect to send.

It is, however, essential that the solution not be myopically focused on refreshing Warehouses in response to artifact push events. We know webhooks will inevitably be used for other things as well and we will not want myopic choices made here and now to force compromises or breaking changes when adding such features.