Preparing runwayctl to migrate to Mimir

Part of gitlab-com/gl-infra/scalability#3448 (closed).

What

• helper function to add auth header
• default tenant
• helper function to add custom tenant
• confirm vault keys to use gitlab-com/gl-infra/scalability#3448 (comment 1909154145)
• retrieve config from environment variables
• impersonated client code removed, basic auth client added

Why

We are migrating from Thanos to Mimir. We need to extra code to authenticate, as the new setup requires a tenant header and a basic authentication token.

Issue: team#245