Support GCS default credentials for authentication

[arbourd]

Google Cloud allows users to authenticate as a service account automatically. This is the proffered method to authenticate when running in/on GCE or GKE.

Changes that would need to be made to support keyfile-less authentication:

• Make keyfileName optional https://github.com/tweedegolf/storage-abstraction/blob/6b823ea0e0dc83ecb9954cb19c6ebbd87ae417a8/src/AdapterGoogleCloudStorage.ts#L67
• If the keyfile was not provided, nor were application default credentials, an exception will be thrown
• Pull project ID directly from this.storage: this.storage.projectId, instead of parsing it from file