Introduce a new type wrapping PartialAddress, PublicKey and AztecAddress

[benesjan]

As @spalladino pointed out here it would be valuable to introduce a new type wrapping the aforementioned types because these 3 values are needed to send user a note and for this reason it's common that they are "appearing" together.

Possible names are:

1. ExtendedAddress,
2. CompleteAddress,
3. FullAddress.

I am personally ok with any of these names.

I think the introduction of this type will be a bit confusing so explain the class docs why the type is needed.

• Introduce the type,
• make getAccounts return the new type instance and drop the getPublicKeyAndPartialAddress function,
• write docs explaining why the type is needed.