[mimir-continuous-test]: Support authentication

[Logiraptor]

Is your feature request related to a problem? Please describe.

mimir-continuous-test currently supports using a tenant ID only. It would be nice to support Basic Authentication and Bearer token authentication. These are both supported in Grafana Enterprise Metrics, so they would be a necessary step to run continuous testing against a GEM cluster in certain configurations. It would also be useful for any users who have configured any other authenticating gateway in front of OSS Grafana Mimir.

Describe the solution you'd like

Add three additional flags:

-basic-auth-username
-basic-auth-password
-bearer-token

These would be used in the continuoustest Client struct to perform the various HTTP requests similar to how tenant-id works today.

Describe alternatives you've considered

We could alternatively only support deploying mimir-continuous-test behind the authentication layer, but this is limiting especially in highly regulated deployments.

Additional context

This feature would enable customers with enterprise auth enabled in a Grafana Enterprise Metrics cluster to use the helm test command as a smoke test. Currently this is only possible if you have auth.type set to trust.