feat: support read and sync of Register data types without providing a wallet

[happybeing]

The current APIs (e.g. ClientRegister::sync()) require a wallet when read-only sync and even write-sync of an existing Register is free.

Since there will no doubt be a use for apps to just inspect or update existing registers this seems an unnecessary requirement. A wallet is only needed if the operation needs to create a Register, so I think everywhere that the developer does not need or expect to create a Register, provision of a wallet should either not be possible, or at least be optional.

This could be made explicit with API naming such as sync() and sync_or_create().