curl-impersonate support

curl-impersonate allows to fight with TLS fingerprinting, a technique used to prevent scraping (here's an example offering from Cloudflare: https://developers.cloudflare.com/bots/concepts/ja3-fingerprint).

I think supporting it here boils down this:

[ ] add impersonate-chrome and impersonate-ff optional features, changing the searched library config to libcurl-impersonate-chrome and libcurl-impersonate-ff respectively (#483)
[ ] add support for static-curl feature with impersonate features (using curl-impersonate instead of upstream; not necessarily needed)
[ ] expose curl_easy_impersonate in curl-sys crate (limited to impersonate features; docs: https://github.com/lwthiker/curl-impersonate#advanced-usage)
[ ] expose curl-impersonate specific opts (on impersonate features)
[ ] re-expose curl_easy_impersonate in curl::easy::Easy2 (and curl::easy::Easy?)

alexcrichton / curl-rust