PubsubIO.Read support for timestamp parsing

[damccorm]

Hi,

I'm repeatedly facing situations where I need to assign timestamp from PubSub message body and it's not possible via Pubsub.Read.

Having some method like withTimestampFn(SerializableFunction<ReceivedMessage, Long\>) would be very helpfull.

BEAM-53 mention such feature:

??getting timestamp from an arbitrary lambda in arbitrary formats rather than from a message attribute in only 2 formats.??

But looks that it was never implemented.

Imported from Jira BEAM-9728. Original Jira may contain additional context.
Reported by: bushkovsky.