ssh: add the ability to enable compression (for SCP/SFTP)

The required low-level logic was already available as part of
`libssh2` (via `LIBSSH2_FLAG_COMPRESS` `libssh2_session_flag()`[1]
option.)

This patch adds the new `libcurl` option `CURLOPT_SSH_COMPRESSION`
(boolean) and the new `curl` command-line option `--compressed-ssh`
to request this `libssh2` feature. To have compression enabled, it
is required that the SSH server supports a (zlib) compatible
compression method and that `libssh2` was built with `zlib` support
enabled.

[1] https://www.libssh2.org/libssh2_session_flag.html

Ref: https://github.com/curl/curl/issues/1732
Closes https://github.com/curl/curl/pull/1735

include/curl/curl.h

@@ -1788,6 +1788,9 @@ typedef enum {
   /* bitmask of allowed auth methods for connections to SOCKS5 proxies */
   CINIT(SOCKS5_AUTH, LONG, 267),
 
+  /* Enable/disable SSH compression */
+  CINIT(SSH_COMPRESSION, LONG, 268),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;