Support attaching files to results

[DanielPBak]

testrail's python API seems to support attaching files: https://www.gurock.com/testrail/docs/api/reference/attachments

(add_attachment_to_result)

It would be nice if the pytestrail plugin included hooks for this API, so that we can publish files with our test results.

[VStoilovskyi]

@DanielPBak Hello there! Looks like this issue is a duplicate of #137