Add API calls for file download, history and diffs

[wonder-sk]

• download a file of a project at version particular version (using latest version if no version was specified
  e.g. download_file(project_name, file_path, output_filename, version=None)

• get history of when a particular file of a project was modified (for "versioned" files - i.e. geopackages)
  e.g. get_file_history(project_name, file_path) and it would return a list (or dictionary?) with version numbers + timestamp + author

• get diff between two versions of a file - also just for "versioned" files - all partial diffs would get concatenated to a single diff (using GEODIFF_concat() from new geodiff)
  e.g. get_file_diff(project_name, file_path, output_diff, version_from=None, version_to=None)