Create BOM for grpc-java

[ejona86]

A BOM provides versions for multiple libraries to aid keeping the versions consistent.

See https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-bom/pom.xml and googleapis/google-cloud-java#2307

[ejona86]

CC @garrettjonesgoogle

[ejona86]

There's now a question whether this would actually be helpful. Moving to unscheduled.

[ejona86]

BOMs still align the version between different artifacts, even if the user wasn't aware they were using a particular artifact (transitive dependency). Thus it would be useful for us to have.

[garrettjonesgoogle]

gax-java (which also uses Gradle) has successfully published a BOM: googleapis/gax-java#607

[saturnism]

quick ping. this would be really handy right now.. :D