MEDIUM: http: Add new 'set-src' option to http-request

MEDIUM: http: Add new 'set-src' option to http-request

This option enables overriding source IP address in a HTTP request. It is
useful when we want to set custom source IP (e.g. front proxy rewrites address,
but provides the correct one in headers) or we wan't to mask source IP address
for privacy or compliance.

It acts on any expression which produces correct IP address.