Adds an example plugin implementing an mTLS-enabled binary cache store
(https+mtls:// scheme) using client certificates for authentication.
Darwin fix: don't link liblix* into plugins (host resolves symbols at
runtime via dynamic_lookup). Explicitly link curl so it binds to
Nix-store libcurl, not /usr/lib/libcurl. This prevents the plugin's
curl_easy_setopt calls from operating on the wrong libcurl instance.
Test portability: BSD sed -i wrapper, OpenSSL -sha256 for cert signing,
redirect test server output to log file.
Change-Id: I652b987d3ac45e31df50ff4ba1f523294438c2b6
