Allow adding custom header to all requests

[kbrooks]

At my company, we have a self-hosted sentry instance behind an IAP authentication layer, which makes it impossible to use the Sentry CLI as it exists today. On the old sentry CLI we did this:

export SENTRY_PROJECT="your-project-name"
export CUSTOM_HEADER="SC-LCA-1: ..."

Support for this behavior was added in this PR getsentry/sentry-cli#1077

This doesn't seem to be supported on the new CLI which doesn't support a custom header or a header parameter on the sentry auth login command

sentry auth login  -H "SC-LCA-1: ..."

and the plain command fails with

 ERROR  Failed to initiate device flow                                                                                                                                                          auth.login 1:01:31 PM
  Endpoint: /oauth/device/code/
  Invalid IAP credentials: empty token

Can we add support for custom headers to the new cli?

[BYK]

@kbrooks thanks for the issue! Looking into how we can get this done.

[BYK]

@kbrooks LMK if the design in #761 would work for you and I'll merge it