Allow external systems to send messages to build targets
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Description

Allow external systems to send messages to build targets

Summary:
Ref T1049. Allows external systems to send a message to a build target. The primary intended use case is:

You make an HTTP request to Jenkins.
The build goes into a "waiting" state.
Later, Jenkins calls harbormaster.sendmessage to report that the target passed or failed.
The build continues as appropriate.

This is deceptively complicated because:

There are a lot of race concerns. We might get a message back from an external system before it even responds to the request we made. We want to make sure we process these messages no matter when we receive them.
These messages need to be sent to a build target (vs a build or buildable) because we'll get into trouble with parallelization later on otherwise (Jenkins is told to do 3 builds; we can't tell which ones failed or what overall state is unless the message are sent to targets).
I initially thought about implementing this as a separate "Wait for a response from an external system" build step. This gets a lot more complicated for users once we do parallelization, though. Particularly, in the case where you've told Jenkins to do 3 builds, the three "wait" steps need to know which target they're waiting for (and jenkins needs to know some unique identifier for each target). So this pretty much boils down to a more complicated, more error-prone version of using target PHIDs.

This makes the already-muddy Build UI a bit worse, but it needs a general clarity pass anyway (it's showing way too much uninteresting data, and should show a better summary of results instead).

Test Plan: