Added delegates to rest dispatch and ability to return result type directly from iface methods.

Preceded by: master
Followed by: #586

The gist of this change is explained in the javadoc of [ResourceDelegate](https://github.com/ArcBees/GWTP/pull/585/files#diff-18)

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Merged all PRs into this one. Will hold until the CI confirms build success.
To not have Guava as a dependency of `rest-dispatch-shared`. And yes it should be `stringBuilder.append(separator).append(element);`