[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.

CHANGELOG.md

@@ -6,6 +6,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Keys
+#### Changed
+- Renamed `DerivableKey::add_metadata()` to `DerivableKey::into_descriptor_key()`
+#### Added
+- Added an `ExtendedKey` type that is an enum of `bip32::ExtendedPubKey` and `bip32::ExtendedPrivKey`
+- Added `DerivableKey::into_extended_key()` as the only method that needs to be implemented
+
 ### Misc
 #### Added
 - Added a function to get the version of BDK at runtime