Support writing to dynamic subject with NATS SINK

[priestprophetking]

Nats supports fine grained subjects which can handle 10's of millions of subject names. This makes using identifiers in subject names very powerful.

https://docs.nats.io/nats-concepts/subjects

It would be great if arroyo could dynamically write the subject into the sink perhaps by using a reserved column in the sink table.

CREATE TABLE results ( count BIGINT NOT NULL, time TIMESTAMP NOT NULL, subject TEXT ) with ( type = 'sink', connector = 'nats', servers = 'localhost:4222', stream= 'alerts', format = 'json' );