Add long timers to Python

[andystaples]

Adds support for Long Timers in Python
Refer to this PR which added support for long timers in NodeJS, this PR is the same implementation in Python: https://github.com/Azure/azure-functions-durable-js/pull/340/files

[davidmrdavid]

GH asked for a review from me automatically, so took a peek. By no means exhaustive. This looks mostly in the right track, just left some questions. Might be worth removing me as a code owner.

Also - please triple check, in the extension and reference PRs from Hossam that I sent Chris, that there are no other "mandatory" features that the SDK needs to implement when opting in the "ooproc protocol v3". I seem to recall a that maybe "long polling callhttp" was also a requirement, or maybe it was just exposed by the extension in v3 as well. In any case, please do a check to the extension code to confirm there are no bad consequences if no changes to callhttp are made.

I also recommend adding a few unit tests here, particularly around the exception cases.