Elegant permissions handling for multiple permissions stores

Created by: emchap

(I know there has been conversation about this elsewhere—documenting at @unknwon's request! But see also: https://github.com/sourcegraph/sourcegraph/issues/14736)

Feature request description

An ability to support permissions from multiple code hosts or multiple instances of a single code host more elegantly than our current solution.

Is your feature request related to a problem? If so, please describe.

This has shown up among our customer base as a need, and though we do support it the current solution is unwieldy.

Describe alternatives you've considered.

Current support: https://docs.google.com/document/d/1e5cHQdF8Y7vCJFeVknLMkK0Tp0XckpshedIiD5SxXO8/edit?usp=sharing

Additional context

This came up due to a prospective customer's need, but has come up elsewhere in the current customer base as well.