Create messages database/backend that the validator is willing to sign

[aaronbuchwald]

Implement a WarpBackend to be used by the precompile onAccept and the signature request handler:

type WarpBackend interface {
	AddMessage(ctx context.Context, unsignedMessage *teleporter.UnsignedMessage) error
	GetSignature(ctx context.Context, messageHash ids.ID) ([]byte, error)
}