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/options.go

@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"github.com/owncloud/ocis/proxy/pkg/user/backend"
 	"net/http"
 	"time"
 
@@ -26,6 +27,8 @@ type Options struct {
 	HTTPClient *http.Client
 	// AccountsClient for resolving accounts
 	AccountsClient acc.AccountsService
+	// UP
+	UserProvider backend.UserBackend
 	// SettingsRoleService for the roles API in settings
 	SettingsRoleService settings.RoleService
 	// OIDCProviderFunc to lazily initialize an oidc provider, must be set for the oidc_auth middleware
@@ -165,3 +168,10 @@ func TokenCacheTTL(ttl time.Duration) Option {
 		o.UserinfoCacheTTL = ttl
 	}
 }
+
+// UserProvider sets the accounts user provider
+func UserProvider(up backend.UserBackend) Option {
+	return func(o *Options) {
+		o.UserProvider = up
+	}
+}