This patch implements osclientcerts for macOS.
Because the SDK we build with isn't recent enough, some of the functions we
need aren't guaranteed to be available. To handle this, we load the Security
framework at runtime and attempt to locate the symbols we need. If this
succeeds, then operation proceeds as normal. Otherwise, the module will report
that there are no certificates/keys available.

Differential Revision: https://phabricator.services.mozilla.com/D59957

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extra : moz-landing-system : lando