Enhance the endpoint to list existing files of the configuration server with the possibility to specify a version

[mariusoe]

The configuration server offers the possibility to list all existing folders and configuration files of the workspace branch (which is used by the UI for example). In this case, the current status of the workspace branch is always used as a basis.

In this ticket this endpoint should be extended so that a specific version can be specified (based on the commit ID) to which the result refers. This should make it possible to get the content of the workspace branch from a specific version (commit). It should be possible to specify the ID using a request parameter called version.

Example: /api/v1/directories?version=e3004bed2cf2ba75a4d49d5383919223be58bc5f

By default, the most current version of the workspace branch should be used (for example, if no version parameter is specified).

It should also be possible to fetch the current content of the configuration server's live branch. For this purpose, a special verison id can be reserved, e.g. `live', which causes the content of the current live branch to be returned.

Example: /api/v1/directories?version=live