Add web hook support

[dylan-bourque]

Consider extending the Perseus server to support web hooks that could be invoked when a module's dependencies are added/updated. This idea came up in an internal discussion within CrowdStrike but seems like it could be generally useful.

The minimal implementation would be to send a fire-and-forget HTTP POST to an unauthenticated endpoint, or one using basic auth, with a JSON payload that contains the module being updated along with its dependencies. We could even reuse the protojson encoding of the existing UpdateDependenciesRequest message.

More advanced authentication for web hooks should be investigated but may be outside of MVP scope for this feature.