feat: Implement SubscriberShutdownSettings

[michaelpri10]

Implements subscriber shutdown options for the Java client library. This provides more control over the shutdown process:

• The subscriber stream is closed immediately.
• If the behaviour is set to NACK_IMMEDIATELY, then all buffered messages are nacked, and the timeout specifies how long to wait for them to go through.
• If the behaviour is set to WAIT_FOR_PROCESSING, then the remaining message machinery continues until the timeout is almost up, and then it switches to NACK_IMMEDIATELY.
• If the timeout is zero, everything closes down immediately, as quickly as possible.

[feywind]

The logic looks good! I don't know as much about the Java library specifics, but I didn't see anything weird.