Add Gnat.Jetstream.API.Object.Watcher

[jcreager]

To compliment Gnat.Jetstream.API.KV.Watcher this library should support the ability watch objects for changes. This is currently implemented in the nats cli as nats object watch <bucket name>.

@mmmries from an implementation perspective, what do think makes the most sense:

• Refactor lib/gnat/jetstream/api/kv/watcher.ex in some way that supports objects and kv (they are both actions on streams and very similar).
• Use lib/gnat/jetstream/api/kv/watcher.ex as a starting place for lib/gnat/jetstream/api/object/watcher.ex knowing there will be a fair amount of similarity in the implementation.

My personal instinct is to use lib/gnat/jetstream/api/kv/watcher.ex as a starting place and not worry about the similar implementations, but I'd like to hear your thoughts on this.