Add ability to work with realm level roles

[wtrocki]

We need to add docs to readme about to do it

keycloak-connect-graphql/src/KeycloakContext.ts

Lines 38 to 39 in b60c900

	* * `hasRole('realm:admin')` will check the logged in user has the admin realm role
	* * `hasRole('some-other-app:admin')` will check the loged in user has the admin realm role in a different application

It looks like current lib requires users to create roles on the server client and it is not working with cluster-level roles.