Permit customizing the header name for the IAP jwt

This creates a new configuration parameter `jwtHeader` for the gcp-iap
auth provider. This allows setting a custom header to look in for the
IAP issued JWT.

Signed-off-by: Adam Kunicki <kunickiaj@gmail.com>

docs/auth/google/gcp-iap-auth.md

@@ -26,6 +26,7 @@ auth:
   providers:
     gcp-iap:
       audience: '/projects/<project number>/global/backendServices/<backend service id>'
+      jwtHeader: x-custom-header # Optional: Only if you are using a custom header for the IAP JWT
 ```
 
 You can find the project number and service ID in the Google Cloud Console.