Config: add common config for object storage

[trevorwhitney]

Is your feature request related to a problem? Please describe.
When configuring loki, I need to provide configuration for a cloud object storage backend in multiple places. This is tedious as that config is often the same, as I usually only use one object storage for a given loki installation.

Describe the solution you'd like
Add an object_store section to the common config section in the loki config