Maven archetype generation should not depend on parent pom files

Currently, the sdks/java/maven-archetype projects shell out to maven in order to generate the archetypes. The project pom files use parent pom's, and thus create a dependency on maintaining the existing Maven pom files.

We should break the dependency from this project to the rest of the pom files such that they can eventually be retired.

Some ideas for breaking the dependency:

1. Move the necessary logic inherited from the parent pom's directly into the archetype project pom files and remove the parent dependency. There shouldn't be much that the archetype build actually depends on.
2. Convert the archetype generation to gradle. I'm not sure whether a native Gradle plugin exists, but the logic for jar generation should be simple.

There has been some conversation [on the dev list](https://lists.apache.org/thread.html/c52a34d2e7cbb59f5c2d27a8ccebe542a21a214aab30bfb6f6f6f9cc@%3Cdev.beam.apache.org%3E)

Imported from Jira [BEAM-4382](https://issues.apache.org/jira/browse/BEAM-4382). Original Jira may contain additional context.
Reported by: swegner.