Allow clients to namespace data

With multiple SDK flavours, we need to allow clients to namespace data when storing, retrieving and subscribing to data changes. We already see the potential issue when using the same pubkey in mainnet and testnet when using the data-sync. With multiple SDK flavours potentially using the same pubkeys with different data structures this issue could get worse.

I propose we allow the namespace to be optionally set by the client, and default to something like `"nodeless:mainnet"` internally, then each SDK flavour client sets their own namespace like `":"`