Add capability introspection to url operations framework

[mih]

There are a number of issues that point out that capabilities of different URL handlers is different (and maybe will remain different). This installs a challenge for client-code. Users can configure handlers to use and client-code need to adjust to such choices. Without knowing of a, e.g., deletion can be forced (#454), or performed recursively, complex try-except logic is the only way to mitigate.

I propose to have handlers report their capabilities. This could be done with a simple enum that defines particular features (and the docs would communicate exactly what their definition is), and each handler would expose a set of feature labels.

With this setup in place, client-code can determine whether it is possible to perform one or the other algorithm, or whether individual aspects can be left to optimized implementations in specialized handlers.