Add support for mTLS

Is your feature request related to a problem? Please describe. Some servers require a client certificate. Currently, there seems to be no way to provide a client certificate to hetty.

Describe the solution you'd like Implement a way to provide a client certificate to hetty for specific hosts. Ideally, there would be a button in the UI that allows selecting a .p12 file, and a textbox for the target hosts (e.g. *.example.com). When hetty establishes a connection with the target host, it should provide the client certificate to the server.

Describe alternatives you've considered I have been using Burp Suite so far, but would prefer to use hetty due to it being open source.