Allow per request SSL client certificate options

[cplussharp]

For testing mTLS connections it would be easier to have the SSL client certificate options next to the request and not per host in the settings. So you could have multiple requests to the same host but with different mTLS certificates.

This would also make the .http/.rest file independent of the workspace.

I already uploaded a pull-request, how it could be done for cUrl ( #808 ), but I'm not sure how it could be done for .http?
Maybe in the comment part before the request, like it is suggested in issue #680 ?