Derive security header report url from DSN

I know that this could be done somewhat trivially by parsing the DSN URL in my own code (assuming the format is stable).
However, it'd be convenient to have this as part of the SDK, so one could do just get it via something like `sentry_sdk.csp_report_url`.

先閱讀 SDK 如何表示及解析 DSN，然後檢視提議的 `sentry_sdk.csp_report_url` 存取模式。定義預期的衍生 URL，並加入涵蓋測試以顯示它可從 DSN 取得；該 issue 未指定來源檔案或測試位置。

由索引模型根據 Issue 內容生成。