Add the setRequestMetadata() method to allow integration to services like Mixpanel

[DanProudfoot]

Hi

I recently tried to implement Mixpanel and Branch in a Capacitor app, but I couldn't get attribution to work from Branch -> Mixpanel. Mixpanel's docs say that the device id they provide should be sent to Branch in the setRequestMetadata function which is included in the iOS and Android Branch SDKs but not exposed by this library

Branch / Mixpanel docs pertaining to the java/ObjC calls used to make the request: https://help.branch.io/using-branch/page/mixpanel#4-pass-idmetadata-to-branch

The Cordova version of this project includes this function: https://help.branch.io/developers-hub/docs/cordova-phonegap-ionic#link-data-mixpanel-integration

[chiemekailo]

@DanProudfoot hello, it appears this issue was never addressed. Did you find a way around it?