BEP: Support longer streams on non building commands

While the build event protocol is mainly targeted for commands that actually
build, a minimal stream is generated for all commands. For commands like "query",
it is desirable that the stream contains the full output of the command.
To achieve this, introduce an optional second event indicating the end of the stream;
note that the NoBuildEvent has to come before the payload answer as the
experimental UI uses this event to determine the transition to the payload answer
that is passed through unchanged.

RELNOTES: None.
PiperOrigin-RevId: 159977543

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