cta-dcache integration as native subdirectory
Summary
This is merge request introduces dCache frondend to CTA. The full development history is preserved as-is. In contrast to MR !186 (closed) the code is added as a native sub-directory.
After multiple iteration the module can be updated to be a more generic gRPC frontend and adopted by other user facing service, e.g. EOS. Nonetheless, current focus is dCache, as EOS developers might need some adjustments to messages.
References
resolves #1222 (closed)