Support for key selection

[divarvel]

Currently Biscuit::parse expects a single private key, but it should be possible to pass a function that selects a private key based on the optional root_key_id value.

This might be tricky to achieve with the current rust API, since it expects a closure for key selection. It is possible to pass a closure from JS to rust, but then it's trick to turn the closure result JsValue into a PublicKey. A possible solution would be to provide specific helpers in the rust lib that deserialize the outer proto layer and provides the key id, before continuing.