Generate the pom.xml as part of the build

@cbeams - We were not sure if we wanted to do this or not, but I put together a pull request to see what you thought.

//cc @philwebb @rstoyanchev

Previously the pom.xml was only generated when doing maven deployments.
This is nice since the metadata can be derived from the Gradle build.
However, there have been a issues that have come from using this
approach (i.e. SPR-10218).

Since the pom.xml files are so critical, it is nice to include these in
version control so that we can perform a diff each time they change.
This commit introduces a new task that generates the pom.xml files and
updates the .gititnore to include the pom.xml files.