Add a way in sdk to provide the nostr object to the sdk for nip07

[rramaa]

• Currently, we take the nostr object from window. We assume that the sdk will only be used in browser environments and user will have the nostr extension
• We need to provide a setter function that will set the nostr object locally in the sdk. By default it will be defaulted to window.nostr. If the client wants to set the nostr object, there will be an option to do so