Adds conformance test

[bg451]

This is a prototype to show how we could test each client library against our conformance suite using google cloud build.

To test this out locally, install the local build runner viagcloud components install cloud-build-local and then run cloud-build-local --dryrun=false --config=cloudbuild.yaml .. The test should fail since the ruby lib doesn't implement the binary carrier :)

[jmacd]

Is this correct:
Each client library will copy the first 6 lines of this file into its own.

There's probably some value in standardizing the way this is done to allow re-use of a yaml fragment, but if it's this tiny, probably not very much value.

[bg451]

I believe the way to standardize is to essentially bundle these into it's own container so that it runs in one step.

[jmacd]

(Should this target depend on having the conformance tested bundled by another rule? N/m I see that's difficult in Ruby--none of the other rules do this.)

I was wondering if we could shorten this snippet by just using go run github.com/lightstep/conformance/runner.go ruby conformance_test/client.rb, maybe.

I take it this runs locally, assuming the runner binary is installed? This makes me want to rename runner to be more descriptive, like conformance-runner.

^^^ I was thinking of ways to make this Makefile work for local development, too, without requiring a docker container to run this test. The PATH above only applies to the docker container case, which makes me think there's a better way to do this. Sigh. Makefiles. Suck.