Add support for post-assessment Webhook about magtape HOT 3 OPEN

In the past we'd talked about a gRPC-based sidecar service to enable offloading of the alerting work from the primary MagTape.

1. create a protocol buffer definition for message/alert "type" and service (proto3)
2. implement sidecar which implements the service (go?)
3. implement the python client functions to build a message/alert from a policy result and offload messaging to the gRPC service

If we still fancy exploring this route I think we can begin work on the protocol buffer piece and try to get an idea what the API's input and output may look like.

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Sounds good. Assigning to you @ilrudie . Reach out whenever you want to sync on this.

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Would be nice if this could become a sort of pluggable interface allowing users to define the specific endpoint, a templates request, and expected response.

If this is done, the existing Slack alerts may be able to be migrated to this pluggable interface

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