Bug 1712837 - introduce ipcclientcerts to allow client certificates to work with the socket process r=rmf,kershaw,necko-reviewers,ipc-reviewers,nika,jschanck

This patch introduces ipcclientcerts, a PKCS#11 module that the socket process
can load to get access to client certificates and keys managed by the parent
process. This enables client certificate authentication to work with the socket
process (particularly for keys stored outside of NSS, as with osclientcerts or
third-party PKCS#11 modules).

Differential Revision: https://phabricator.services.mozilla.com/D122392