Please error, or at least warn, when a managed type is included in INetworkSerializeByMemcpy

Is your feature request related to a problem? Please describe.

My coworker was unfamiliar with INetworkSerializeByMemcpy and the FixedStringNNNBytes types, and added a string field to an existing INetworkSerializeByMemcpy struct. This caused me great headaches (it made it into a publicly released patch), trying to track down segfault heap corruption that only showed up in very specific multiplayer circumstances (the string was normally null, which probably masked the issue).

Describe the solution you'd like

Please cause a build error when a managed type (a class, a string, or the like) is included in an INetworkSerializeByMemcpy type. I cannot think of any use case where it would be valid or intended to serialize a managed GC pointer by memcpy. At least a warning if nothing else.

It doesn't have to be perfect, even just checking for string would be lovely - string specifically is such a massive footgun, it's so easy to accidentally include and think it'll work fine. The fact it crashes later on via unrelated segfault heap corruption bringing down the editor, instead of a C# Exception, is especially rough.

首先定位 INetworkSerializeByMemcpy 的定義及其驗證或序列化測試。追蹤實作該介面的型別在編譯期間或執行階段如何受到檢查，然後為 string 欄位和其他受管理欄位新增涵蓋；當此類型別產生建置錯誤或警告，而不是進行不安全的 memcpy 序列化時，即表示完成。