Feature: cumulative ack

[jeffgrunewald]

Currently the Pulsar websocket API does not support cumulative ack
There is an issue (apache/pulsar/issues/5979) to add support for this functionality.
Once that feature is supported by Pulsar websockets, Stargate should provide the option to ack cumulatively.

Proposal:

• by default, all stargate ack messages are singular
• if an optional configuration value is passed to a consumer (ack_type: :cumulative) then ack messages append the necessary field to the ack payload to tell the cluster the ack should be treated cumulatively.

This depends heavily on the final implementation of the upstream project feature.