Java: support multiple versions per release

[chingor13]

Java publishes packages together and different artifacts are versioned independently. We already have precedent in google-cloud-java to manage all versions in a versions.txt file at the root directory and to be able to annotate places to replace throughout the README.md and pom.xml files.

This PR replaces the current utilites/bump_versions.py and utilities/replace_versions.py scripts in google-cloud-java so they can be used with other repositories like gax-java.

The workflow is now:

1. What type of release? (MINOR|patch|snapshot)
2. Bump versions.txt? (Y/n)
3. Update versions in pom.xml files? (Y/n)
4. Edit release notes
5. Create release branch? (Y/n)
6. Create PR? (Y/n)

See googleapis/google-cloud-java#3777

This should make releasetool work for all of google-cloud-java, gax-java, google-auth-library-java, google-api-java-client, google-http-java-client, google-oauth-java-client.

[chingor13]

@theacodes @busunkim96 As I'm not a pythonista yet, should I break the internal classes in here into separate files?

[garrettjonesgoogle]

What would the usage look like in gax-java?

[chingor13]

I haven't looked into the gradle release process yet.

[garrettjonesgoogle]

I don't think it needs to be built into the gradle release process yet; the question was more about how the direct commands would look.

[chingor13]

@garrettjonesgoogle I meant looking into how gradle handles release and snapshot versioning. The releasetool workflow should be identical.

[garrettjonesgoogle]

@chingor13 Ok. So what commands would I run manually to invoke the bump & replace-versions logic?

[chingor13]

If you want to separate the steps, you can do so by replying no to selective prompts. Both scenarios would start with releasetool start.

If you want to just bump, you can say yes to updating versions.txt, and no to updating the pom files.

If you want to just replace the versions, you can say no to updating versions.txt and yes to updating the pom files.

[garrettjonesgoogle]

I'm cool with this

[chingor13]

This is ready to go whenever.