Support Proxy-Authorization header in kargo CLI

[steve-marmalade]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

Provide a mechanism to supply the kargo CLI with a Proxy-Authorization header.

Motivation

I have deployed the kargo API behind GCP IAP so that it's only accessible to admins. Visiting the web-app is straightforward, since GCP intercepts the request and then forwards me to the UI once I log-in.

I would like to do the same via the CLI, and IAP supports the Proxy-Authorization header so as not to interfere with an application's use of the Authorization header.

Suggested Implementation

Adding a flag to kargo login either allowing arbitrary headers or a --proxy-authorization token would be amazing.