`CreateAccountUseCaseImpl`: consolidate account provisioning

[s373r]

Expectations

• Nuke the heck out impl Into<Account> for AccountConfig
• Extend AccountConfig to carry optional private key
• Have CreateAccountUseCase accept AccountConfig
• The only place generating and saving the keys should be CreateAccountUseCaseImpl
• The only place that supplies AccountConfig:id should be the Web3WalletAuthenticationProvider

Based on #1642 (comment)