Support file upload for releases in API

Description

Gitlab has the useful release feature, which allows to upload files to releases / tags [1].

Unfortunately the corresponding API functions do not include file upload [2].

Proposal

From a backend perspective, a good way to do this is via https://gitlab.com/gitlab-org/gitlab-ce/issues/60630

The API functions

• create-a-new-release
• update-a-release

should have following additional parameters (compare create-new-file-in-repository):

• file_name (optional) - Name of new file. Ex. release-v0.9.3.tar.gz
• file_encoding (optional) - Change encoding to 'base64'. Default is text.
• file_content (optional) - File content

Links / references

• [1] https://gitlab.com/help/workflow/releases.md
• [2] https://docs.gitlab.com/ce/api/tags.html#create-a-new-release