Supporting runtime style

Even though `vtshaver` is not designing for runtime style, we still want to make efforts to make things better.

Now we have two experimental ideas. This ticket is to track the process of support runtime style.

1. **Style specific retain**, we want to make some special marks in style specific, to mark which properties in a particular layer we want to use, then the vtshaver will keep the certain properties.

2. **Query properties**, vtshaver well accept a param to determine which properties the user want to use, then return the shaved tiles+the certain properties. For the client side, every time user wants to change the style, the client can access tile with the certain properties in query(like `vector.pbf?styl=&keep={"country_label" : ["name_ru", "name_de", "name_es"]}`).

cc w/ @springmeyer

The issue names no files, tests, or entry points, so begin by locating the existing style-optimization flow in vtshaver. Compare the two proposed approaches—style-specific retention and a query parameter—and define which selected properties must remain in the shaved tiles before implementation; done means runtime style changes can use the required properties.

Written by the indexing model from the issue text.