Allow passing new RealtimeField as string

[assylias]

If a new realtime field is exposed by the Bloomberg API, it can't be access until the RealtimeField enum is updated.

Allow to pass a realtime field as a string to the SubscriptionBuilder.

[Kurry]

Yes! Both a Collection<String> fields and String field, the number of real time fields are in the hundreds if not thousands, and so I think this makes the most sense. I have to do this in my own fork. Also, adding the ability to add subscription options would be helpful. I like this change because really, you could just get rid of the SubscriptionHolder class. Not only should you allow people to pass new RealtimeField, you also should allow them to pass strings for subscription options.