Function get_register_available_route 

pub(crate) async fn get_register_available_route(
    __arg0: State<State>,
    __arg1: ClientIp,
    body: Args<Request>,
) -> Result<Response>

GET /_matrix/client/v3/register/available

Checks if a username is valid and available on this server.

Conditions for returning true:

• The user id is not historical
• The server name of the user id matches this server
• No user or appservice on this server already claimed this username

Note: This will not reserve the username, so the username might become invalid when trying to register