Plug uprops into icu4x-datagen and add examples using icu_testdata::get_provider()

[iainireland]

Following up on the minimal uprops provider work in issue #882 and issue #883, we should:

1. Add support in icu4x-datagen (somewhat complicated by the fact that compiling and running an ICU4C tool is necessary)
2. Add example in icu_uniset::props

[sffc]

1. Add support in icu4x-datagen (somewhat complicated by the fact that compiling and running an ICU4C tool is necessary)

No, we should not depend on ICU4C being run. The idea is that ICU4C ships a zip file with all the properties pre-exported. For the time being, icu4x-datagen should take a path to that directory, with instructions on how to generate it manually, and eventually (later this quarter or in Q4) you will be able to download that directory as a zip file from GitHub.

[iainireland]

Zip file export is being added to ICU4C here: unicode-org/icu#1857.

[sffc]

We need to add uprops to icu4x-datagen and then follow up shortly thereafter (or in the same PR) with crate-level and property-level examples in icu_properties.