fix: Fix typings for eventwebhook.d.ts PublicKey (sendgrid#1351)

Installing @sendgrid/eventwebhook in a project using typescript cannot properly compile because starkbank-ecdsa does not provide any typings and none are available in DefinitelyTyped project either.
Right now the only way to use the module is to ourselves provide a definition of starkbank-ecdsa OR patch-package @sendgrid/eventwebhook

I've decided to change the import statement with a minimal interface of PublicKey which should be enough to use the package properly in a typescript project.

Technically speaking, @types/node is also required because you use Buffer in the types, but a typescript project without it would be weird so I think we can let that one as-is.

packages/eventwebhook/src/eventwebhook.d.ts

@@ -1,4 +1,10 @@
-import {PublicKey} from "starkbank-ecdsa";
+// From starkbank-ecdsa, minimal interface of PublicKey
+declare class PublicKey {
+  toString(encode?: boolean): string;
+  static fromPem(pem: string): PublicKey;
+  static fromDer(der: string): PublicKey;
+  static fromString(key: string): PublicKey;
+}
 
 declare class EventWebhook {
     /**