[PM-9598] Introduce integrations (#10019)

Factor general integration logic out of the forwarder code.

- Integration metadata - information generalized across any integration
- Rpc mechanism - first step towards applying policy to integrations is abstracting their service calls (e.g. static baseUrl)

Email forwarder integrations embedded this metadata. It was extracted to begin the process of making integrations compatible with meta-systems like policy.

This PR consists mostly of interfaces, which are not particularly useful on their own. Examples on how they're used can be found in the readme.

libs/common/src/tools/integration/rpc/rest-client.ts

@@ -0,0 +1,68 @@
+import { ApiService } from "@bitwarden/common/abstractions/api.service";
+import { I18nService } from "@bitwarden/common/platform/abstractions/i18n.service";
+
+import { IntegrationRequest } from "./integration-request";
+import { JsonRpc } from "./rpc";
+
+/** Makes remote procedure calls using a RESTful interface. */
+export class RestClient {
+  constructor(
+    private api: ApiService,
+    private i18n: I18nService,
+  ) {}
+  /** uses the fetch API to request a JSON payload. */
+  async fetchJson<Parameters extends IntegrationRequest, Response>(
+    rpc: JsonRpc<Parameters, Response>,
+    params: Parameters,
+  ): Promise<Response> {
+    const request = rpc.toRequest(params);
+    const response = await this.api.nativeFetch(request);
+
+    // FIXME: once legacy password generator is removed, replace forwarder-specific error
+    //   messages with RPC-generalized ones.
+    let error: string = undefined;
+    let cause: string = undefined;
+
+    if (response.status === 401 || response.status === 403) {
+      cause = await this.tryGetErrorMessage(response);
+      error = cause ? "forwarderInvalidTokenWithMessage" : "forwarderInvalidToken";
+    } else if (response.status >= 500) {
+      cause = await this.tryGetErrorMessage(response);
+      cause = cause ?? response.statusText;
+      error = "forwarderError";
+    }
+
+    let ok: Response = undefined;
+    if (!error && rpc.hasJsonPayload(response)) {
+      [ok, cause] = rpc.processJson(await response.json());
+    }
+
+    // success
+    if (ok) {
+      return ok;
+    }
+
+    // failure
+    if (!error) {
+      error = cause ? "forwarderError" : "forwarderUnknownError";
+    }
+    throw this.i18n.t(error, rpc.requestor.name, cause);
+  }
+
+  private async tryGetErrorMessage(response: Response) {
+    const body = (await response.text()) ?? "";
+
+    if (!body.startsWith("{")) {
+      return undefined;
+    }
+
+    const json = JSON.parse(body);
+    if ("error" in json) {
+      return json.error;
+    } else if ("message" in json) {
+      return json.message;
+    }
+
+    return undefined;
+  }
+}