Add kotlin-jpa-specification-dsl package to source

This is a partial revert of 8f9e6431. That commit added a dependency for jpaspecificationdsl, a package that consisted of just one file: this one. However, the repository it was included in (jcenter) has been removed: see github and jcenter.

Since it's just a single file, we are including it here. To keep the rest of the code intact, we're keeping the original package name and structure.