Multiple column key and "keyless" pivot models

還沒有人認領這個 Issue。

**Is your feature request related to a problem? Please describe.**
Currently there is no way to have a model without an id field. For pivot models and in some other cases you want to have multiple identifier fields instead of a single id field.

**Describe the solution you'd like**
Adding `id = true` to multiple @Column annotations should result in all fields being used as the model identifier.

此 issue 將 @Column 註解和模型識別碼處理確定為入口。首先定位處理 id=true 的 Java 程式碼，並檢視現有的識別碼行為；完成的標準是支援多個識別碼欄位和不含 id 的模型，同時維持目前的行為不變。

由索引模型根據 Issue 內容生成。