[Cashtab] Create and test new wallet creation and validation functions

Summary:
T3445

Cashtab's wallet creation needs to be cleaned up before we can get wallet handling into the new standardized state and db management.

Duplicate existing functionality of Cashtab wallet creation engines in a function outside of the useWallet hook. Create a unified isValidCashtabWallet function that will replace isLegacyMigrationRequired and isValidStoredWallet functions.

Create new createCashtabWallet function in wallet/ that matches current Cashtab behavior. Add an integration test to confirm this.

None of the created functions are implemented in this diff.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15604