Configurable file name

[hallvictoria]

Description

The Python worker reads the function app entry point from an app setting called PYTHON_SCRIPT_FILE_NAME. The default value for this setting will still be "function_app.py". Customers can signal to the worker to read from a file not called "function_app.py" by setting this variable to the new name of their file.

Added unit and e2e tests for this app setting by creating a function app file not called "function_app.py" and testing it.

Also added unit tests for 2 invalid V2 app scenarios: no top level function app instances and no inputs

Fixes #https://github.com/Azure/azure-functions-pyfx-planning/issues/72

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PR information

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• [ ] There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• [ ] If applicable, the PR references the bug/issue that it fixes in the description.
• [ ] New Unit tests were added for the changes made and CI is passing.

Quality of Code and Contribution Guidelines

• [ ] I have read the contribution guidelines.

[codecov[bot]]

Codecov Report

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

Comparison is base (16c109a) 85.11% compared to head (9347e42) 85.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1323 +/- ## ========================================== + Coverage 85.11% 85.84% +0.72% ========================================== Files 35 35 Lines 1955 1971 +16 Branches 370 371 +1 ========================================== + Hits 1664 1692 +28 + Misses 217 210 -7 + Partials 74 69 -5 ``` | [Flag](https://app.codecov.io/gh/Azure/azure-functions-python-worker/pull/1323/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-worker/pull/1323/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | `85.79% <100.00%> (+0.73%)` | :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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