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/flagset/flagset.go

@@ -256,6 +256,14 @@ func ServerWithConfig(cfg *config.Config) []cli.Flag {
 			Destination: &cfg.EnableBasicAuth,
 		},
 
+		&cli.StringFlag{
+			Name:        "account-backend-type",
+			Value:       "accounts",
+			Usage:       "account-backend-type",
+			EnvVars:     []string{"PROXY_ACCOUNT_BACKEND_TYPE"},
+			Destination: &cfg.AccountBackend,
+		},
+
 		// Reva Middlewares Config
 		&cli.StringSliceFlag{
 			Name:    "proxy-user-agent-lock-in",