Add support for using a TLS client certificate with 'erc-tls' (bug#47788)

Add support for using a TLS client certificate with 'erc-tls' (bug#47788)

* lisp/erc/erc-backend.el (erc-session-client-certificate): New
buffer-local variable storing the TLS client certificate used for the
current connection.
(erc-open-network-stream): Use open-network-stream instead of
make-network-process, and pass any additional arguments to it.
(erc-server-connect): Add an optional client-certificate argument
that if present is passed with the :client-certificate keyword as part
of the arguments to erc-server-connect-function.
* lisp/erc/erc.el (erc-open): Add new optional client-certificate
argument, set it as erc-session-client-certificate, and pass it along
to erc-server-connect.
(erc): Clarify documentation string with respect to the full-name
argument.
(erc-tls): Add new client-certificate keyword argument and pass it in
the direct call to erc-open (instead of going through erc).
(erc-open-tls-stream): Pass any additional arguments (such as
:client-certificate) to open-network-stream.  Also allow overriding
:nowait if desired.

* doc/misc/erc.texi: Add documentation for erc-tls, including the new
:client-certificate argument.

* etc/NEWS: Announce the change.