Add config option to control auto Camel-Case feature when previewOption is set to exchange

[wujunchuan]

Add a extension config option to control the custom request header Camel-Case

For example there is a HTTP request 👇

@HOST=http://localhost:8888
GET {{HOST}}/collect/mockindex/ HTTP/1.1
auth: "Coll 2.bQk0e2sNddsmNdNdYcaTPszg"
lang: "zh"

Current

GET http://localhost:8888/collect/mockindex/ HTTP/1.1
Auth: "Coll 2.bQk0e2sNddsmNdNdYcaTPszg"
Lang: "zh"
User-Agent: vscode-restclient
Accept-Encoding: gzip

The custom HTTP headers auth and lang will auto uppercase its first letter

Except

GET http://localhost:8888/collect/mockindex/ HTTP/1.1
auth: "Coll 2.bQk0e2sNddsmNdNdYcaTPszg"
lang: "zh"
User-Agent: vscode-restclient
Accept-Encoding: gzip

[Huachao]

@wujunchuan I have fixed the issue and will always preserve the outgoing request headers case.

[Huachao]

@wujunchuan you can try the latest version 0.23.0 to verify it

[wujunchuan]

@Huachao works well, it will not puzzle me again now