Allow user provided function to extract custom timestamp from payload in pubsubIO

Currently the PubsubIO runner only allows the caller to set a custom timestamp if the timestamp is defined in the attributes of the message. This can be problematic when the user does not control the publisher. In such a case, proper windowing of data requires the timestamp to be pulled out of the message payload.

Since a payload could have arbitrary data, the user would have to provide a Function() that would extract the timestamp from the payload:

PubsubIo.Read.timestampLabel(Function extractor);

Imported from Jira [BEAM-1936](https://issues.apache.org/jira/browse/BEAM-1936). Original Jira may contain additional context.
Reported by: berkoben.

Start by locating the PubsubIO timestampLabel API and its runner implementation, then review BEAM-1936 for additional context. Define how a user-provided function extracts timestamps from message payloads and verify that the resulting timestamps support correct windowing.

Written by the indexing model from the issue text.