Generalize CMake for examples to use external librustls

[cpu]

As a follow-up to #506 it would be nice to remove the need for the 3rd party https://github.com/cpu/rustls-ffi-test repo for smoke-testing built artifacts.

The existing client/server examples would be perfect for this, except that the CMakeLists.txt they use unconditionally build librustls from scratch using cargo-c pointed to output into the cmake build root.

It would be better if we generalized this so that if an existing librustls was available (e.g. from being discovered via pkg-config) the build would prefer to use that copy of the lib instead of building it anew. This would let us use the test binaries with the pre-built artifacts during the testing phase of the binary artifact release CI process.