Add account creation / account load

[Apolixit]

Utilize Substrate.NET.Wallet to implement the functionality for creating and loading wallets for players in the game. The account information should be saved in a local text file.

Target :

1. The system should allow a player to create a new wallet using Substrate.NET.Wallet, by showing him a random 24 words mnemonic sentence.
2. The system should allow a player to load an existing wallet using mnemonic or JSON + password.
3. Upon wallet creation or loading, the JSON account information (without the password), should be saved in a local text file.