Add optional "serviceName" property to all service methods

[kiwi-josh]

Resolves: https://github.com/martinkasa/capacitor-secure-storage-plugin/issues/51

Optionally allow serviceName to be provided when calling the methods, defaulting to the existing "cap_sec" if not provided to maintain backwards compatibility

[de-dan]

+1 but

• [ ] Android implementation missing
• [ ] I get that in the iOS implementation your new serviceName property is actually used as serviceName for the KeychainWrapper, but in Web the property is used for the storage entry key. So I would suggest a more global naming like prefix or keyPrefix