[lib][native] Add nonceTimestamp to SIWEResult

Summary:
SIWEResult is a type only used on native. It's what SIWEPanel returns after a successful SIWE.

For ENG-7665, we want to know when the nonce was generated in order to determine whether to allow the user to use the cached SIWE result.

Would normally put @varun on the review here, but going to ask @inka instead because he is out.

Test Plan:
I tested (or will test) this diff stack as follows:

1. Be in a multi-keyserver environment, testing SIWE with iOS simulator
2. Do a SIWE and then wait 2 minutes to let the nonce expire in the following screens:
   • FullscreenSIWEPanel for an account that doesn't exist yet when the new registration flow is disabled
   • FullscreenSIWEPanel for an account that doesn't exist yet when the new registration flow is enabled
   • FullscreenSIWEPanel for an account that does exist
   • New registration flow for an account that doesn't exist yet (RegistrationTerms)
   • New registration flow for an account that does exist (ExistingEthereumAccount)
3. Make sure there are no duplicate Alerts, that in all cases an Alert is shown, and that the "back" action activates when the user confirms the Alert

Reviewers: inka

Reviewed By: inka

Subscribers: tomek, varun, inka

Differential Revision: https://phab.comm.dev/D12005