Specify filepath in `cobertura_xml_report`, instead of folder

Feature

Currently, the cobertura_xml_report is used as folder path, in which mypy typically only produces a single file cobertura.xml.
I would like to be able to specify a direct output path of the xml file, without having additional folders produced.

Pitch

Using the xml report in a CI requires additional config to archive an extra folder, which is unneeded if a target file can be specified.
Even when specifying something like cobertura_xml_report = "mypy.xml", the output file is still mypy.xml/cobertura.xml.
A potential workaround --cobertura-xml-report=. is unsafe because mypy simply overwrites any cobertura.xml produced by any other tool.

If the use case is too different from the other reporting outputs which all use DIR, please consider allowing this option via toml config.
For instance, something like this:

[tool.mypy]
cobertura_xml_report = {file = "mypy.xml"}
html_report = {dir = "mypy"}

まず、mypy が cobertura_xml_report オプションをどのように処理し、他のレポート出力がパスをどのように解釈しているかを追跡します。要求された直接のファイルパスを、既存のフォルダーの動作および提案されている TOML のファイル／ディレクトリの区別と比較します。設定された XML ファイル名が追加のフォルダーなしで直接書き込まれ、既存のディレクトリの動作が明確かつ安全なままであれば完了です。