Make it possible to use CS3 as accounts backend instead of account-service

Configureable via:
PROXY_ACCOUNT_BACKEND_TYPE=cs3
PROXY_ACCOUNT_BACKEND_TYPE=accounts (default)

By using a backend which implements the CS3 user-api (currently provided by reva/storage) it is possible to bypass
the ocis-accounts service and for example use ldap directly.

Hides user and auth related communication behind a facade (user/backend) to minimize logic-duplication across middlewares.
Allows to switich the account backend from accounts to cs3.

Co-authored-by: Jörn Friedrich Dreyer <jfd@butonic.de>

proxy/pkg/middleware/authentication.go

@@ -122,6 +122,7 @@ func newOIDCAuth(options Options) func(http.Handler) http.Handler {
 // newBasicAuth returns a configured basic middleware
 func newBasicAuth(options Options) func(http.Handler) http.Handler {
 	return BasicAuth(
+		UserProvider(options.UserProvider),
 		Logger(options.Logger),
 		EnableBasicAuth(options.EnableBasicAuth),
 		AccountsClient(options.AccountsClient),