fix: Reverts GetAccount to use IAccount. Adds IAccount to serialization. (#1565)

### Summary

- Fixes usernames not being `Intern`ed
- Reverts methods related to getting accounts from returning `Account` to `IAccount`.
- Makes `IAccount` also `ISerializable`
- Adds `IGenericReader.ReadAccount()` and `IGenericWriter.Write(IAccount)` -> The read method supports the original serialization of username, and using `IAccount.Serial`. The write method only serializes the `Serial`.
- Exposes `ReadStringRaw()` to allow some advanced scenarios.

Projects/UOContent/Misc/ServerAccess.cs

@@ -76,8 +76,8 @@ public static class ServerAccess
             return;
         }
 
-        var acct = Accounts.GetAccount(username);
-        if (acct == null || !acct.Banned && acct.AccessLevel >= AccessLevel.Owner || !acct.CheckPassword(e.Password))
+        if (Accounts.GetAccount(username) is not Account acct
+            || !acct.Banned && acct.AccessLevel >= AccessLevel.Owner || !acct.CheckPassword(e.Password))
         {
             return;
         }