upload local images as attachments for push command

[AenEnlil]

We need to implement option to upload local suite/test-related images to testomat as attachments.

1. User adds files_to_upload section to markdown test/suite block with paths to local files.
2. On push, read each files_to_upload section, encode the referenced files, and include them in the attachments field of the payload, each attachment mapped to its suite or test. files_to_upload section is removed after processing
3. Payload is sent to the server with attachments alongside tests and files OR send attachments in separate request
4. Backend stores the attachments and links them to the corresponding suite/test.(in Sidekiq Job i guess)

Note. Document files_to_upload section

[DavertMik]

Use importing api via dispatch/load