Add support for Descriptions in PropertyStatistics

Building upon the refactorings in f0db935, f8d9fdf and f57c060,
adding support for descriptions on top of labels is fairly simple:

• Add DescriptionConfig subclassing TextConfig
• Add one conditional to parse_config_properties

The Queries endpoint feature is a bit more annoying, hence the
introduction of the TEXT_SELECTOR_MAPPING which will hopefully
go away in a future refactoring.

(the work for the Jinja templates had been done already in b74b35).

Bug: T248788