Support a pre-existing JetStream stream for events

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• The following new options should be made available for starting a lattice observer:
  • Stream name - The name of a pre-existing stream, observer should assume this contains cloudevents from a wasmbus event stream.
  • JS domain - JetStream domain of the pre-existing stream
  • Snapshot every N - Indicates the observer should emit a snapshot event (which will purge pre-existing events from the stream) after every n events.
  • Snapshot every Mins - Indicates the observer should emit a snapshot event after every n minutes. This option is mutually exclusive with the previous option.
• When an observer starts, it will:
  • Create an ephemeral consumer for the indicated stream (if one is provided)
  • Read stored events until NATS indicates the pending event count in the stream is 0.
  • Stop the replay subscription when the event pending count from NATS is 0.
  • Ignore all snapshot instructions during replay

The net result of these changes will make it so that, in the presence of a JetStream event stream for wasmbus.evt.* events, an observer can start at any time and automatically catch up, generating accurate state. It can also short-cut the amount of events it needs to read upon replay through the addition of a new cloud event, the lattice_snapshot_created event.

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Some JS reference - using the reply-to field on the messages we can tokenize the subject and obtain the pending count: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-15.md#jsack