http: add client cert support for HTTPS proxies

http: add client cert support for HTTPS proxies

Git supports performing connections to HTTPS proxies, but we don't
support doing mutual authentication with them (through TLS).

Add the necessary options to be able to send a client certificate to
the HTTPS proxy.

A client certificate can provide an alternative way of authentication
instead of using 'ProxyAuthorization' or other more common methods of
authentication.  Libcurl supports this functionality already, so changes
are somewhat minimal. The feature is guarded by the first available
libcurl version that supports these options.

4 configuration options are added and documented, cert, key, cert
password protected and CA info. The CA info should be used to specify a
different CA path to validate the HTTPS proxy cert.

Signed-off-by: Jorge Lopez Silva <jalopezsilva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>