Allow specifying certificates for the client

[akoidan]

User Story Description

My server does MTLS authentication, so the clients have to provide client cert, ca-cert, and also use their private key.
For this feature I want to generate the preview for different clients (e.g. curl/nodejs) that allows to add certificates to it.
Also it would be glad if we could programatically restrict languages in the ui.

Currently I had to put a hack just to manually watch the dom:

I can work on this feature and make a pull request.