Move pom generation metadata logic out of build_rules.gradle

When preparing a release, we generate pom files for each artifact to be released. Most of the pom structure is filled in automatically, but there is a good deal of metadata that needs to be manually added. The [code for manipulating the XML structure](https://github.com/apache/beam/blob/c78b96a67c0313f44d78262ee46f86aa4866a3aa/build_rules.gradle#L605) is verbose and it would make more sense to actually express it as XML rather than groovy DOM manipulations.

It would be nice to refactor this and clean up the build_rules.gradle file a bit. Some potential ideas for refactoring:
- Check in a "base pom.xml" file pre-filled with metadata, and use this as the source for generated pom manipulations.
- Check in and release a parent pom file which contains all required metadata. Other artifacts can reference it as a parent and then drop the duplicated metadata.
- At the very least, refactor the pom manipulation logic out to a different method or different file.

Imported from Jira [BEAM-4050](https://issues.apache.org/jira/browse/BEAM-4050). Original Jira may contain additional context.
Reported by: swegner.