[keys] Improve the API of `DerivableKey`

[keys] Improve the API of `DerivableKey`

A new `ExtendedKey` type has been added, which is basically an enum of
`bip32::ExtendedPubKey` and `bip32::ExtendedPrivKey`, with some extra metadata
regarding the `ScriptContext`.

This type has some methods that make it very easy to extract its content as
either an `xprv` or `xpub`.

The `DerivableKey` trait has been updated so that the user now only has to
implement a method (`DerivableKey::into_extended_key()`) to perform the
conversion into an `ExtendedKey`.

The method that was previously called `add_metadata()` has now been renamed
to `into_descriptor_key()`, and it has
a blanket implementation.