[Spanner Backup & Restore] Codify pontem's version

[mirons-google]

As we create releases for pontem, we should update (as part of the commits in the release) a change to a global string in pontem that tracks the version.

Then src/main/java/com/google/cloud/pontem/Util.java#L81 should reference that global version string so things between it and util.java stay in sync.

[EricBeach]

Is there a pipeline, etc. that we can use to run tests for this against a release candidate? (I'm looking for a way to automate this instead of just human dependent and I don't see one looking around in GitHub).

[nagarkar]

Eric, is this still relevant? Is this something an external contributor could do (label with help-wanted/good-first-issue)?

[EricBeach]

I am not sure how relevant it is since we now have so many different subprojects.

[nagarkar]

I'll close it in that case. Well probably need a new cross platform way to track releases and doing it just for one tool probably isn't the right default option anymore. Michael feel free to reopen if you disagree.