Consolidate java-templates folder into rest of module

[nwithan8]

Description

• Move EasyPost.java into module rather than as a generated source
• Mark immutable variables as final
• Remove unused apiVersion global variable

Testing

• All unit tests pass as expected

Pull Request Type

Please select the option(s) that are relevant to this PR.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

I assume that Java magically knows where to find these values even though the location of the file changed?

Does this file have anything to do with the templating-maven-plugin we use in this repo by chance?

In Java, the file path doesn't really matter, it's the namespace.

However, after more investigation, it seems we need to keep this template file. The Templating Maven Plugin compiles any template files (like this one) at compile time, extracting template information (in this case, the project.version value from the pom.xml file). If we move this file, it will no longer be considered a template, and the version number will not be extracted properly.

[Justintime50]

Aha. Can we please instead add a comment to the file explaining its purpose and how it works with the plugin so future travelers know? That plus the removal of the apiVersion var should be good enough.

[Justintime50]

@jchen293 let's review this ASAP. I want to cut a release of the Java lib with this included so we can get that getSmartrates fix out.