mendersoftware / mender / 1019656555

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feat: Implement `mender::update::http_resumer`

Implement class to download the Artifact, which will react to server
disconnections or other sorts of short read by scheduling new HTTP
requests with `Range` header.

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests for
an introduction to the feature, and read the specification for more
details.

The user calls _once_ `AsyncCall` with the header and body handlers, and
`DownloadResumerClient` will call back these handlers _once_ (each). At
the user's header handler, the Reader returned by `MakeBodyAsyncReader`
is a wrapper of the actual Body reader that is taking care of the resume
of the download, calling the `AsyncRead` finalized handler only after
the download is fully completed (with potential resumes) and or/the
resuming times out.

The validation of the `Content-Range` header and the cases for the unit
tests are heavily inspired by the legacy client. See:
* https://github.com/mendersoftware/mender/blob/<a class=hub.com/mendersoftware/mender/commit/d9010526d35d3ac861ea1e4210d36c2fef748ef8">d9010526d/client/update_resumer.go#L113
* https://github.com/mendersoftware/mender/blob/d9010526d35d3ac861ea1e4210d36c2fef748ef8/client/update_resumer_test.go#L197

Ticket: MEN-6498
Changelog: None

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

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