Add request deadline to OperationContext subclasses

[mjameswh]

Description

• Adds request_deadline field to OperationContext and CancelOperationContext. Resolves Expose Request Deadline on Operation Context #29.
• Marked all properties on OperationContext and StartOperationContext as kwonly.

[VegetarianOrc]

Because this deadline is related to requests and not the operation, should we consider putting this in the cancellation object I added in #31? Might help to keep the options related to task cancellation together.

We can also get around the ordering issue and keep the default by making it a keyword only field, which seems fairly reasonable to me. Though if we add it to the task cancellation object we'd also avoid it.

    request_deadline: Optional[datetime] = field(default=None, kw_only=True)
    """
    Get the deadline for the operation handler method. Note that this is the time by which the
    current _request_ should complete, not the _operation_'s deadline.
    """

[mjameswh]

I moved the request deadline to OperationContext and marked it as kw_only, as suggested.

[cretz]

Not sure this field needs a default, it never had one before

[mjameswh]

I guess my concern was more that the comment says it is optional, but the field's type isn't. So the field's default factory was to consolidate those two contradicting facts, but I can alternatively change the doc or make the field's type Optional. No strong opinion on those options.

Also, tbh, I only have little prior knowledge of Python dataclasses, so it is totally possible that I'm misunderstanding something here.

With that in mind, do you maintain I should remove the default?

[VegetarianOrc]

I think we should change the doc to remove the word optional and leave the default value off. For users that are implementing an operation handler, they will still always have a Mapping to inspect. For users implementing a worker (such as the Temporal worker), they'll be required to provide a Mapping which will then encourage them to either consciously default to an empty dict or think about how to handle headers in a more robust way.

[cretz]

I guess my concern was more that the comment says it is optional

The comment says that it's optional for the caller to have ever set these per Nexus spec, it has nothing to do with whether the field type is optional for the creator of this object (which is us). There is a difference between what is optional for a caller in the Nexus spec and what is optional for us when instantiating a class.

[cretz]

Suggested change

	request_deadline: Optional[datetime] = field(default=None)
	request_deadline: Optional[datetime] = None

Don't need the field part, None is immutable/acceptable here

[cretz]

Suggested change

	callback_url: Optional[str] = field(default=None)
	callback_url: Optional[str] = None

Same as other place