[Feature]: Implement the `SubAccountManagement` protocol

[jonbarrow]

Checked Existing

• [X] I have checked the repository for duplicate issues.

What feature do you want to see added?

An implementation of the SubAccountManagement protocol.

Why do you want to have this feature?

Not used by NEX, but is provided by Rendez-Vous. Will be useful to people making non-NEX game servers. Also makes the library more feature complete.

Any other details to share? (OPTIONAL)

This protocol has no documentation. Needs to be reverse engineered from a compatible game.