Added missing methods and fixed existing ones with file download in job artifacts API

[akashdotsrivastava]

Closes #586

• Fixes the existing file download methods to correctly serve FileResponse
• Adds missing methods for downloading single file from artifacts for a job, and for a branch/tag from https://docs.gitlab.com/ee/api/job_artifacts.html#get-job-artifacts

[akashdotsrivastava]

@NARKOZ
Hi. If it's okay, would you mind adding the hacktoberfest-accepted label to this PR before you approve, if/when you approve? Would be great if this becomes a part of my hacktoberfest contributions 😊

[NARKOZ]

@akashdotsrivastava sure. Thanks for the contribution.