Enhance the endpoint to fetch the file contents of the configuration server with the possibility to specify a version

[mariusoe]

The configuration server offers the possibility to fetch the content of configuration files of the workspace branch (which is used by the UI for example). Currently, 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 resulting filte content refers. This should make it possible to get the content of a file in a specific version (commit). It should be possible to specify the ID using a request parameter called version.

Example: /api/v1/files/example.yml?version=e3004bed2cf2ba75a4d49d5383919223be58bc5f

By default, the content of the file located in the latest version of the workspace branch should be used (for example, if no version parameter is specified).

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

Example: /api/v1/files/example.yml?version=live

This enhancement should only refer to the read access of the file content (GET /api/v1/files/...). Write access to specific versions should not be allowed!

[mariusoe]

closed by #910