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/config/config.go

@@ -115,6 +115,7 @@ type Config struct {
 	PolicySelector        *PolicySelector `mapstructure:"policy_selector"`
 	Reva                  Reva
 	PreSignedURL          PreSignedURL
+	AccountBackend        string
 	AutoprovisionAccounts bool
 	EnableBasicAuth       bool
 	InsecureBackends      bool