add proxy middleware to fetch user roles for the token context

[butonic]

after the account has been determined we need to fetch the roles the account has. should be a dedicated roles middleware.

open design question:

• should we fetch all roles or can we reduce the scope by looking at the request destination / other properties? if subsequent services need to know about roles in other scopes they can fetch them themself?
• should we mint all roles into the token?

Open PR: owncloud/ocis-proxy#70

Current Implementation

The middleware fetches all role assignments for the authenticated user and mints their roleIDs into the x-access-token.

Services performing permission checks can then use the RoleManager from ocis-pkg to keep a self-updating local cache of those roles and run permission checks against that RoleManager.

[IljaN]

Should we mint all roles into the token?

Not "all" but the once assigned to the user.