fix: support ordering by key for multi queries

[Primevenn]

fix: support ordering by key for multi queries

Fixes #629

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[Primevenn]

@googlebot I signed it!

[chrisrossi]

@Primevenn This PR looks great. Thank you for submitting it.

In order to accept this PR we'll need unit test coverage for the new code. There are also some linting errors that can be cleaned up by running black. (nox -e blacken)

Once you can run nox locally and not get any errors, you should be about good to go.

[Primevenn]

@chrisrossi Thanks for the review!

As you suggested i've appended flat_path to the 2 lines under the if statement that handles ordering by Entity __ key __ and have moved the other if statements inside the else block.

These 2 extra if statements are necessary for ordering by keyProperty since helpers._get_value_from_value_pb() will return a Key object for key_value fields.

I've also added unit test coverage for these 2 scenarios and ran black.

[chrisrossi]

Excellent. Thank you!