implement REST client backend

[ThomasWaldmann]

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[ThomasWaldmann]

I played a bit with the already existing REST server of restic via borgbase.com, but there are some differences, so that won't "just work":

• it has no mkdir/rmdir and does not adhoc-create needed dirs, but creates all needed dirs at repository creation time on the server
• list: it does nesting and directory recursion on the server, while borgstore does only flat dir listings in the backend and does nesting and recursion in Store, above the backend, on the client side. I did it like that to reduce backend complexity (and only implement that directory recursion once) and also to limit the result size of the directory listing (N/2**16 instead of N).
• it enforces key == sha256(content), so .store can't work with arbitrary keys (like HMAC(plaintext))
• .destroy (delete a repo) didn't work, maybe a borgbase.com specific feature so users don't accidentally kill their repos but have to use the web UI for that?

[ThomasWaldmann]

@m3nu @buengese ^^