pkg/cloud: ResumingReader incorrectly forgets to call .Close() on its ReadCloser

[knz]

Describe the problem

Since go 1.17.6, it has become critical to call .Close() on the Body field of a http Response object, for otherwise we get goroutine leaks.
(see #74657, golang/go#50652 and #75136)

Thanks to #75136, we're now doing a pretty good job of doing so in most cases, except for the following:

• cloud/httpsink/(*httpStorage).ReadFileAt calls openStreamAt which returns a http.Response
• that Response's Body is then passed as an io.ReadCloser to cloud.NewResumingReader,
• which, in turns does not call .Close() when the Reader is reset in Read(), when RetryOnErrFn returns true

(NB I've found it hard to read through the implementation of the methods of cloud.ReadResumer, it was expecting some kind of disciplined centralization of the invariants on the ReadCloser - maybe there's some improvement to make there)

cc @adityamaru for triage

[blathers-crl]

cc @cockroachdb/bulk-io

[adityamaru]

Thanks for digging into this! I think we can add the Close() call before we nil it out over here https://github.com/cockroachdb/cockroach/blob/a9077697a77aa41f1cd09d3173616be37e85e1bf/pkg/cloud/cloud_io.go#L234-L234, I can send a PR.