Support header for queries from control plane to external prometheus

Feature Request

What problem are you trying to solve?

We have an external prometheus server and we deploy prometheus agents to scrap all target pods' metrics inside the cluster and push to remote prometheus. So write should be fine.
But for reading metrics, the clients need to use the API token in the header to query the metrics.
curl -H "Authorization: Bearer ${API_TOKEN}" https://${REMOVE_PROMETHEUS_URL}/api/v1/query

Right now there is no way to add the token to linkerd control plane. https://github.com/linkerd/linkerd2/blob/main/viz/metrics-api/cmd/main.go#L51

How should the problem be solved?

The library that Linkerd is using supports adding headers using RoundTripper https://github.com/prometheus/client_golang/pull/817/files#diff-152f0c0b2d5899f52f5578b6fc8c6d48d3ea8afabb1f7a59b1676c2730298e23R167

What do you want to happen? Add any considered drawbacks.
Expose this token config to the deploy interface.

Any alternatives you've considered?

Is there another way to solve this problem that isn't as good a solution?

How would users interact with this feature?

If you can, explain how users will be able to use this. Maybe some sample CLI
output?

Start in viz/metrics-api/cmd/main.go and review the referenced prometheus client_golang RoundTripper support. Trace how the control plane's external Prometheus URL is configured and exposed through the deploy interface. Done means users can configure a token and queries include it in the Authorization header.