Added support for local storage

Initial implementation of local storage

Main Changes:

• Added support for a DataStore protocol for persisting Matrix objects to a custom defined object store. API consists of two protocols:
  • DataStore: A storage protocol which can persist supported types to storage, which supports both async and non-async access
  • Storable: An associatedtype indicating the key format used by the DataStore for object identification.
• Currently supported DataStore objects that can be persisted include Session, User, Room, Credentials, ClientEvent, ClientEventWithoutRoomId.
• Implemented a GRDBDataStore package class for local SQL DB support
• Matrix sessions asynchronously serializes itself after every sync response if provided a DataStore
• Added some unit tests for sanity testing GRDBDataStore implementation

Additional Changes:

• Matrix:
  • WellKnown: Added Equitable conformance and public initializer
  • NativeImage: Added Encodable and pseudo decodable conformance (extension/subclassing not really possible for implementing full Codable protocol)
• Session: Added Codable conformance
• User: Added Codable conformance
• Room:
  • Added Codable conformance
  • Changed session, type, and version constant fields to mutable fields
• InvitedRoom: Added Codable conformance
• MXC: Fixed/added encoding implementation
• Added type alias for DeviceId
• Credentials:
  • Updated implementation to match the response from the /login API endpoint
  • Added Equitable conformance
  • Added public initializer
• UnsignedData:
  • Extracted UnsignedData type as a single type instead of having multiple types with the same implementation (e.g. ClientEvent.UnsignedData and ClientEventWithoutRoomId.UnsignedData are separate types, although implementation is identical)
  • Added public initializer
• Added public initializers to all event types
• Fixed RoomAvatarContent coding keys