Please add the ability to specify the serviceName

[kiwi-josh]

I have an existing cordova app that we are migrating to capacitor.

The previously cordova plugin we were using allowed the serviceName to be specified when reading/writing data.

However this plugin looks like the serviceName is hardcoded to cap_sec here, which means we cannot migrate to this plugin.

The ideal outcome would be to allow the serviceName optionally be passed into the methods, and to fallback to cap_sec if its not supplied (to be backwards compatible).

[kiwi-josh]

PR to resolve this: #82