Allow custom request / response modifications

[mmatczuk]

Proposal:

User provides a JS file that contains one or more of functions

• ModifyRequest
• ModifyResponse
• ModifyProxyRequest
• ModifyProxyResponse

We add that to proxy as a Modifier.

Goja provides good interoperability between JS and Go, see ToVelue. Maybe we could raw htp.Request pointer but this needs more thought.