Persist SIWE backup secrets to SQLite after succesfull registration process

Summary:
This differential saves generated backup secrets in registration context which then persists it into SQLite when the regiatration is complete and database is
correctly re-created.

Test Plan:

1. Add the following gist: https://gist.github.com/marcinwasowicz/dfd0f0c213e662d9c91289a5ee083820. It creates a button that enables to print siwe backup secrets to the console.
2. Execute registration flow.
3. Tap the button. See siwe backup secrets in the console.
4. Log out. Don't kill the app!
5. Log in to the same SIWE account.
6. Tap the button ensure that there is no backup secret in the db.

Reviewers: tomek, inka, atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11638