Implemented resumable downloads

[noxer]

Summary

This PR adds download resume functionality to mmetl.
It verifies the download by adding a small overlap with the existing file and comparing the first 512 bytes.
The HTTP client now identifies itself via the User-Agent "mmetl/1.0".

[-----existing file-----]
                [-------resumed download-------]
                [overlap]

[noxer]

/update-branch

[agarciamontoro]

This is neat, I really enjoyed reading your code 🤓

I left some comments below, mostly nits, but also some about things I don't understand. I have a couple of general comments, too:

• Is it possible to test the functions in download.go? They seem brittle to any future change, so I'd love to have some assertions to ensure behavior is not changed accidentally in the future.
• Can we wrap the errors (when we just do return err) to give them a bit more context to what happened?

Thank you for your work on this, Tim! :)

[agarciamontoro]

Oh, and also: thank you for the diagram in the PR description, it helped me understand the big picture (and now that I'm writing this I'm thinking that it would be good to add it as a comment in the download.go file explaining how things work).

[noxer]

I'll look into the tests. It should be possible to "fake" the request. I'll also look into the error wrapping.

[noxer]

I've added tests and wrapped all returned error

[agarciamontoro]

Cool! Great tests, thank you 🙏