allow header and set header in client

[hunjixin]

need send Authorization header to remote exec cmds. allow header and support set header value in client

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[hunjixin]

@Stebalien

[hunjixin]

anyone can review this pr?

[BigLep]

@hunjixin: are you able to add the test so we can get this merged?

[hunjixin]

@BigLep have tested

[BigLep]

Thanks. We're blocked on finding maintainer review time. It's in the queue though.

[Stebalien]

Merged in https://github.com/ipfs/go-ipfs-cmds/pull/226 (addressing my review). Thanks!