[feature] Feature bundled downloads

[St4NNi]

This PR adds a first draft for "bundled" downloads and a "bundler" service / API described in #72.

Changes:

• Add aruna/api/bundler/services/v1/bundler_service.proto with a publicly facing service that allows users to initiate bundles for multiple objects.
• Add internal bundler services that are used to communicate between the ArunaServer and a new bundler program.
• The Bundler exposes a small webserver that can be used to retrieve data directly from associated storage backends. It should be kept in close proximity to the data-proxy backend and will use a similar interface to make it pluggable to the same backends as the dataproxy.

[das-Abroxas]

Looks like a good start for a first implementation regarding the bundled downloads.