Backport of cli: set content length on operator api requests into release/1.3.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #14634 to be assessed for backporting due to the inclusion of the label backport/1.3.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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http.NewRequestWithContext will only set the right value for Content-Length if the input is *bytes.Buffer, *bytes.Reader, or *strings.Reader 0.

Since os.Stdin is an os.File, POST requests made with the nomad operator api command would always have Content-Length set to -1, which is interpreted as an unknown length by web servers.

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