Use common interface to fetch secrets in HTTP client config

[TheSpiritXIII]

Part 1 for secret provider support within common. See: https://groups.google.com/g/prometheus-developers/c/WKOej_pnhXg

This change consolidates fetching secrets across all authorization types. Authorization types which previously had different structs per secret type now have a single type.

The unification helps with adding secret manager support which part 2 will include. There are two ways we could do this.

1. With a secret "reference" (or "handle"), e.g. a name to a secret which is configured somewhere else. This PR implements this approach: Add support for secret refs via a secret manager #572
2. Inline. This would not be backwards-compatible. The only way I could think of serialization to work with the type defined outside of this package (secret provider configs would be part of the main prometheus repository since they're plugins), is to use a generic secret config type.