Add Durable Decorators directly to Python library

[davidmrdavid]

This PR attempts to provide an implementation of the Durable Functions decorators (for the V2 prog model) within the Python library itself. The trick is to simply expose the right interface but defer the implementation to the Durable Functions SDK.

This PR does just that. In particular, it:

• Adds the DF decorator interfaces to the Python library. This will enable us to implement the DF templates for the V2 programming model with ease, as all DF triggers and bindings will now be exposed from the FunctionApp object.
• It does not make the durable functions SDK a "prod" dependency of the library. The customer needs to bring the durable functions SDK within their app for this to work. If they do not depend on it, we throw a friendly error asking them to bring the DF SDK.
• It makes the DF SDK part of this package's dev-dependencies. This allows us to write unit tests for this behavior.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.16%. Comparing base (31f3d42) to head (330cd1d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #207 +/- ## ========================================== + Coverage 93.10% 93.16% +0.06% ========================================== Files 56 56 Lines 3247 3277 +30 Branches 666 667 +1 ========================================== + Hits 3023 3053 +30 Misses 137 137 Partials 87 87 ``` | [Flag](https://app.codecov.io/gh/Azure/azure-functions-python-library/pull/207/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Azure/azure-functions-python-library/pull/207/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | `93.13% <100.00%> (+0.06%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure#carryforward-flags-in-the-pull-request-comment) to find out more.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.16%. Comparing base (b85eb7a) to head (330cd1d).

:exclamation: Current head 330cd1d differs from pull request most recent head 8894236. Consider uploading reports for the commit 8894236 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #207 +/- ## ========================================== + Coverage 93.10% 93.16% +0.06% ========================================== Files 56 56 Lines 3247 3277 +30 Branches 666 667 +1 ========================================== + Hits 3023 3053 +30 Misses 137 137 Partials 87 87 ``` | [Flag](https://app.codecov.io/gh/Azure/azure-functions-python-library/pull/207/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Azure/azure-functions-python-library/pull/207/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | `93.13% <100.00%> (+0.06%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure#carryforward-flags-in-the-pull-request-comment) to find out more.

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[davidmrdavid]

@gavin-aguiar : are we good to merge this?