Add functionality so that protocols can do custom keepalive on their
connections, when an external API function is called.
Add docs for the new options in 7.62.0
Closes#1641
curl_mime_encoder() is operational and documented.
curl tool -F option is extended with ";encoder=".
curl tool --libcurl option generates calls to curl_mime_encoder().
New encoder tests 648 & 649.
Test 1404 extended with an encoder specification.
For easier sharing with CMake. The contents were reformatted to use
two-space indent and expanded tabs (matching lib/Makefile.common).
Ref: https://github.com/curl/curl/pull/1288
