MEDIUM: proxy/http_ext: implement dynamic http_ext

MEDIUM: proxy/http_ext: implement dynamic http_ext

proxy http-only options implemented in http_ext were statically stored
within proxy struct.

We're making some changes so that http_ext are now stored in a dynamically
allocated structs.
http_ext related structs are only allocated when needed to save some space
whenever possible, and they are automatically freed upon proxy deletion.

Related PX_O_HTTP{7239,XFF,XOT) option flags were removed because we're now
considering an http_ext option as 'active' if it is allocated (ptr is not NULL)

A few checks (and BUG_ON) were added to make these changes safe because
it adds some (acceptable) complexity to the previous design.

Also, proxy.http was renamed to proxy.http_ext to make things more explicit.