feat: Use PermissionReaders to determine available permissions

These readers will determine which permissions
are available for the incoming credentials.
Their results then get combined in a UnionReader
and authorized in a PermissionBasedAuthorizer

src/authorization/UnionPermissionReader.ts

@@ -0,0 +1,40 @@
+import type { CredentialGroup } from '../authentication/Credentials';
+import type { Permission, PermissionSet } from '../ldp/permissions/Permissions';
+import { UnionHandler } from '../util/handlers/UnionHandler';
+import type { PermissionReader } from './PermissionReader';
+
+/**
+ * Combines the results of multiple PermissionReaders.
+ * Every permission in every credential type is handled according to the rule `false` \> `true` \> `undefined`.
+ */
+export class UnionPermissionReader extends UnionHandler<PermissionReader> {
+  public constructor(readers: PermissionReader[]) {
+    super(readers);
+  }
+
+  protected async combine(results: PermissionSet[]): Promise<PermissionSet> {
+    const result: PermissionSet = {};
+    for (const permissionSet of results) {
+      for (const [ key, value ] of Object.entries(permissionSet) as [ CredentialGroup, Permission | undefined ][]) {
+        result[key] = this.applyPermissions(value, result[key]);
+      }
+    }
+    return result;
+  }
+
+  /**
+   * Adds the given permissions to the result object according to the combination rules of the class.
+   */
+  private applyPermissions(permissions?: Permission, result: Permission = {}): Permission {
+    if (!permissions) {
+      return result;
+    }
+
+    for (const [ key, value ] of Object.entries(permissions) as [ keyof Permission, boolean | undefined ][]) {
+      if (typeof value !== 'undefined' && result[key] !== false) {
+        result[key] = value;
+      }
+    }
+    return result;
+  }
+}