_sandboxremote.py: Support stdout and stderr digests
Support remote execution log forwarding using stdout and stderr digests instead of only supporting embedded stdout and stderr. This also moves log forwarding from SandboxREAPI
to SandboxRemote
to avoid duplicating the log with buildbox-run.
The autotools integration test has been extended to check the build log.
See also BuildGrid/buildbox/buildbox-common!222.