feat(VanityArgs): add --save-path option for custom wallet file path specification

[codeesura]

What 💻

• Enhanced VanityArgs structure to include an optional save_path parameter with a default value, allowing users to specify a custom path for saving generated vanity addresses or use a default file.
• Added WalletData and Wallets structures to serialize wallet information into JSON format for saving.
• Implemented the logic in VanityArgs::run to generate vanity addresses based on specified prefix/suffix and to handle saving these addresses to a file.

Why ✋

• The optional save_path with a default value in VanityArgs makes the tool more flexible, allowing users to either choose a specific file for saving vanity addresses or rely on a default file, enhancing user convenience.
• Introduction of WalletData and Wallets structures for JSON serialization allows for a structured and readable format for storing wallet information, facilitating easier data management and retrieval.
• The updated logic in VanityArgs::run provides a comprehensive solution for generating vanity addresses and managing the storage of these addresses, streamlining the entire process for end-users.

Evidence 📷

Evidence showing the functionality of the new features can be provided upon request. This includes screenshots or screen recordings demonstrating:

[dutterbutter]

Closing as we are now making use of a new re-architected approach on the dev branch. Feel free to re-open against dev.