Ruler external support

[jmoscoso1]

What this PR does: Adds support for custom implementations of pusher and queryable

Example of custom implementation of pusher and queryable being used:

LoadConfig(configFile, expandENV, &cfg)

cfg.ExternalQueryable = querier.NewQueryable(c)
cfg.ExternalPusher = pusher.NewPusher(w)

t, err := cortex.New(cfg)

err = t.Run()

Full code at https://github.com/jmoscoso1/cortex_external_ruler and the docker compose environment where it is run https://github.com/jmoscoso1/Docker_compose_cortex

Which issue(s) this PR fixes: Fixes #4782

Checklist

• [x] Tests updated
• [ ] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]