[SG-515] add fastmail integration for generator (#3318)

* add fastmail integration for generator

* prettier

* introduce forwarder interface and implementations

libs/common/src/emailForwarders/forwarderOptions.ts

@@ -0,0 +1,15 @@
+export class ForwarderOptions {
+  apiKey: string;
+  website: string;
+  fastmail = new FastmailForwarderOptions();
+  anonaddy = new AnonAddyForwarderOptions();
+}
+
+export class FastmailForwarderOptions {
+  accountId: string;
+  prefix: string;
+}
+
+export class AnonAddyForwarderOptions {
+  domain: string;
+}