Removed etag from ObjectMetadata interface (#127)

As per @katfang and noted [here](https://cs.corp.google.com/piper///depot/google3/java/com/google/cloud/eventprocessing/gcs/relay/ObjectMetadataPayload.java?rcl=148419331&l=84), `etag` is not currently included in the `ObjectMetadata` payload.

Author: Jacob Wenger

src/providers/storage.ts

@@ -89,7 +89,6 @@ export interface ObjectMetadata {
   };
   crc32c?: string;
   componentCount?: number;
-  etag?: string;
   customerEncryption?: {
     encryptionAlgorithm?: string,
     keySha256?: string,