Support `--header` for CLI commands to support CloudFlare Access or other identify-aware proxies

## Problem statement

We had a request from someone in our community to add this in order to support using the CLI when Coder is hosted under CloudFlare access. Vault did something similar: hashicorp/vault#12508

## Definition of done

Users can use the CLI and pass arbitary headers.

e.g.
```sh
coder login --header "CF_ACCESS_TOKEN=asdf"
coder ls --header "CF_ACCESS_TOKEN=asdf"
```