feat: Support --header for CLI commands to support proxies

[kylecarbs]

Fixes #3527.

[sreya]

Seems fine but wouldn't this make more sense being integrated in the sdk? You could just add a variadic options parameter to codersdk.New and modify the transport on the HTTP client there.

[kylecarbs]

I didn't want to abstract the client just for this change, because it seems like a weird one.

[sreya]

Based on the issue though isn't the expectation that all commands accept an arbitrary list of headers?

[kylecarbs]

Yup, this is a global flag.

[sreya]

Like I said it's fine but it's just weird we're introducing a new way that every command needs to instantiate a client. It's technically a bug to call codersdk.New directly now in cli. Also if any third party wants to use codersdk as a library they lose out on this functionality and have to write a similar wrapper.

Also unless I'm missing something this PR only implements arbitrary headers for login is there some reason we're not addressing the rest of the commands in this PR?

[kylecarbs]

Nah, this just tests it against login but we could test it against arbitrary commands.

My argument for not exposing this functionality is that it's pretty weird and also trivial for someone else to implement if they want to do so.