[bes] Create{Invocation,Configuration,Target}

First cut of forming Invocation for the initial CreateInvocationRequest
of the ResultStore API.

The RS API says that CreateInvocationRequest must be issued
outside of a batch of UploadRequest, but we will rely on
in interposing process to re-arrange the proto into well-formed
requests, as well as take care of protocol details such as
authentication and resume tokens.
This allows the event output stream from ninja to emit only
one UploadRequest type, which is easier for the proxy process
to consume and unpack.

ResultStore defines resources for invocation, configuration, target,
configuredTarget, and action.  Ninja doesn't have a notion of
configuration, so it suffices to use a single null configuration
for everything.  Initially, we also use a single default target, though
in the future, this could/should be expanded based on positional
arguments.  Actions correspond to ninja's edges, and bear a status
and start/end times.

This partial implementation covers the life cycles (create, update,
finalize) of the whole invocation, the default configuration and target,
and individual actions.

Note: this is completely untested.

Fuchsia-Topic: build-event-stream
Bug: 369980343
Change-Id: I78b3d160f50559ed06c6752d414e9498bb4333cb