Added updatable timer sample.

[mfateev]

What was changed

Added updatable_timer sample based on the Java one.

Why?

It demonstrates how to use workflow.wait_condition with a timeout. How to update the sleep interval is a FAQ.

[cretz]

May also want a simple test on this one

[cretz]

Like our other samples w/ no special install instructions, may want to link to top-level README for "prerequisites"

[cretz]

We have a neat, fairly new feature for @workflow.init that lets you operate on workflow input on the constructor, so can change this to:

Suggested change

	@workflow.defn
	class Workflow:
	def __init__(self):
	self.timer: Optional[UpdatableTimer] = None
	@workflow.run
	async def run(self, wake_up_time: float):
	self.timer = UpdatableTimer(
	datetime.fromtimestamp(wake_up_time, tz=timezone.utc)
	)
	await self.timer.sleep()
	@workflow.signal
	async def update_wake_up_time(self, wake_up_time: float):
	# Deals with situation when the signal method is called before the run method.
	# This happens when a workflow task is executed after a signal is received
	# or when a workflow is started using the signal-with-start.
	await workflow.wait_condition(lambda: self.timer is not None)
	assert self.timer is not None # for mypy
	@workflow.defn
	class Workflow:
	@workflow.init
	def __init__(self, wake_up_time: float) -> None:
	self.timer = UpdatableTimer(
	datetime.fromtimestamp(wake_up_time, tz=timezone.utc)
	)
	@workflow.run
	async def run(self, wake_up_time: float) -> None:
	await self.timer.sleep()
	@workflow.signal
	async def update_wake_up_time(self, wake_up_time: float):

(also can remove the assert self.timer is not None # for mypy in the query)

[mfateev]

Nice! So much clearer!

[cretz]

Recommend return type annotations everywhere

[mfateev]

My Java brain protested this for a constructor :).