Enable user to write a custom code to generate a response

[shubhendumadhukar]

Is your feature request related to a problem? Please describe.
Though with handlebars, user has an extensive capability to write conditions, run loops etc without writing a code, there might be some complex use cases where the user would need to write a custom code to generate a desired response.

Describe the solution you'd like
This can be implemented using a custom handlebar helper which can evaluate the code passed in via mock files and generate a desired response. Final implementation could look like following in a mock file:

HTTP/1.1 200 OK
X-Static-Header: Static-Value

{{code}}
function (request, logger) {
    var name = request.query.name;
    return {
        status: 201,
        headers: {
            "X-Dynamic-Header": "Dynamic-Value"
        },
        body: `{'greeting': 'Hello ${name}'}`
    }
}
{{/code}}

Describe alternatives you've considered
NA

Additional context
NA

[shubhendumadhukar]

Implemented and tested in development branch. Will be available for production release in camouflage-server v0.2.6