feat: Add constant WebID extractor.

Closes https://github.com/solid/community-server/issues/397

src/authentication/UnsecureConstantCredentialsExtractor.ts

@@ -0,0 +1,22 @@
+import { getLoggerFor } from '../logging/LogUtil';
+import type { Credentials } from './Credentials';
+import { CredentialsExtractor } from './CredentialsExtractor';
+
+/**
+ * Credentials extractor that authenticates a constant agent
+ * (useful for development or debugging purposes).
+ */
+export class UnsecureConstantCredentialsExtractor extends CredentialsExtractor {
+  private readonly agent: Credentials;
+  private readonly logger = getLoggerFor(this);
+
+  public constructor(agent: string | Credentials) {
+    super();
+    this.agent = typeof agent === 'string' ? { webId: agent } : agent;
+  }
+
+  public async handle(): Promise<Credentials> {
+    this.logger.info(`Agent unsecurely claims to be ${this.agent.webId}`);
+    return this.agent;
+  }
+}