Provide a BuckEvent Publisher service which emits BuckEvents to an
external server implementation.

Closes facebook#226

Repurposes the RemoteEventSink used to connect to Scribe within Meta internal Buck2 and connects
it to a Bazel Build Event Service over gRPC instead. Adds a translation layer between Buck2
build events and Bazel build events.

BuildBuddy uses pattern expand messages to collect the set of targets to report on.
Buck2 does not have a direct correspondance to the target completed message, for now we collect all
targets for which actions are completed and then emit a series of target completed events before we
close the stream.
Stdout and stderr are only included inline in Buck2 and not as CAS items, so we forward them as
inline data.
Failure details are used by BuildBuddy to display build errors in the UI, so we emit these.