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/PermissionReader.ts

@@ -0,0 +1,20 @@
+import type { CredentialSet } from '../authentication/Credentials';
+import type { PermissionSet } from '../ldp/permissions/Permissions';
+import type { ResourceIdentifier } from '../ldp/representation/ResourceIdentifier';
+import { AsyncHandler } from '../util/handlers/AsyncHandler';
+
+export interface PermissionReaderInput {
+  /**
+   * Credentials of the entity that wants to use the resource.
+   */
+  credentials: CredentialSet;
+  /**
+   * Identifier of the resource that will be read/modified.
+   */
+  identifier: ResourceIdentifier;
+}
+
+/**
+ * Discovers the permissions of the given credentials on the given identifier.
+ */
+export abstract class PermissionReader extends AsyncHandler<PermissionReaderInput, PermissionSet> {}