proxy: Cache the role Mapping for some time

Avoid torturing the settings service with "ListRoles" request for
every incoming request to the proxy.
The role Mapping is refreshed if cached data is older than 5 minutes.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f