[native] use identityGenerateNonce in SIWEPanel component
Summary:
NOTE: SIWE won't work yet if usingCommServicesAccessToken is enabled.
This change just sets the nonce state variable to the value received from the identity service
Depends on D10880
Test Plan: logged the nonce and confirmed that the state variable was set successfully
Reviewers: ashoat, inka
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D10881