feat: add additional supported interfaces

This includes the relevant auth headers and a simplification of several others.

src/authentication/Credentials.ts

@@ -0,0 +1,6 @@
+/**
+ * Credentials identifying an entity accessing or owning data.
+ */
+export interface Credentials {
+  webID: string;
+}

src/authentication/CredentialsExtractor.ts

@@ -0,0 +1,15 @@
+import { Credentials } from './Credentials';
+import { HttpRequest } from '../server/HttpRequest';
+
+/**
+ * Responsible for extracting credentials.
+ */
+export interface CredentialsExtractor {
+  /**
+   * Extracts the credentials found in an HttpRequest.
+   *
+   * @param request - The incoming request.
+   * @returns A promise resolving to the credentials.
+   */
+  extractCredentials: (request: HttpRequest) => Promise<Credentials>;
+}