internal: Add base configuration object
Created by: efritz
This PR introduces an abstract store that has allows an idiom over env.Get
in a way that makes an application's envvars easily searchable and contained in a single place.
This is a bit of an experiment to see if this does buy us some code clarity in application setup. This will be first used in the code intelligence and user-code-execution services. This is a pre-step to the effort tracked in https://github.com/sourcegraph/sourcegraph/issues/14822.