Add a rust representation of disk ledger keys

[Eitu33]

Context

• Disk ledger keys are currently handled as byte vectors, example in get_ledger_part return type
• It would be better to have a rust representation of disk ledger keys

TODO

• [ ] Define the KeyType enum
• [ ] Define the Key type, should contain a KeyType and an Address
• [ ] Implement Ser & Deser
• [ ] Replace occurrences of byte vectors keys in ledger_db.rs, massa-bootstrap client.rs, server.rs with Key's