send-email: add client certificate options

send-email: add client certificate options

For SMTP servers that do "mutual certificate verification", the mail
client is required to present its own TLS certificate as well. This
patch adds --smtp-ssl-client-cert and --smtp-ssl-client-key for such
servers.

The problem of which private key for the certificate is chosen arises
when there are private keys in both the certificate and private key
file. According to the documentation of IO::Socket::SSL(link supplied),
the behaviour(the private key chosen) depends on the format of the
certificate. In a nutshell,

	- PKCS12: the key in the cert always takes the precedence
	- PEM: if the key file is not given, it will "try" to read one
	  from the cert PEM file

Many users may find this discrepancy unintuitive.

In terms of client certificate, git-send-email is implemented in a way
that what's possible with perl's SSL library is exposed to the user as
much as possible. In this instance, the user may choose to use a PEM
file that contains both certificate and private key should be
at their discretion despite the implications.

Link: https://metacpan.org/pod/IO::Socket::SSL#SSL_cert_file-%7C-SSL_cert-%7C-SSL_key_file-%7C-SSL_key
Link: https://lore.kernel.org/all/319bf98c-52df-4bf9-b157-e4bc2bf087d6@dev.snart.me/
Signed-off-by: David Timber <dxdt@dev.snart.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>