fix: Update node subscriber feed params typings

[shanehandley-lived]

What change does this PR introduce?

According to the OpenAPI documentation and testing, the feedIdentifier param is optional when fetching a subscriber's notification feed.

The node client typings have this parameter as required. This updates the node client typings to reflect the API definition, so that the param can be excluded.

Why was this change needed?

The feedIdentifier is an optional parameter, and the API accepts an empty object as the second argument to getNotificationsFeed, but it requires suppressing the following typescript error:

Argument of type '{}' is not assignable to parameter of type 'IGetSubscriberNotificationFeedParams'.
  Property 'feedIdentifier' is missing in type '{}' but required in type 'IGetSubscriberNotificationFeedParams'.ts(2345)
subscriber.interface.d.ts(27, 5): 'feedIdentifier' is declared here.

[p-fernandez]

🌟
Great addition with the tests.