Timers use the input timestamp as the timer output timestamp which prevents watermark progress

By default, the Python SDK adds a timer output timestamp equal to the current timestamp of an element. This is problematic because

1. We hold back the output watermark on the current element's timestamp for every timer
2. It doesn't match the behavior in the Java SDK which defaults to using the fire timestamp as the timer output timestamp (and adds a hold on it)
3. There is no way for the user to influence this behavior because there is no user-facing API

https://github.com/apache/beam/blob/dfadde2d3ee0a0487362dbcca80388fdc2ef2302/sdks/python/apache_beam/runners/worker/bundle_processor.py#L650

We should use the fire timestamp as the default output timestamp.