Add build testing of external plugins

[awelzel]

With #3354 we broke the OpaqueVal API for plugins without noticing. Prior breakage was failing to propagate OpenSSL include headers on OSX.

@ckreibich suggests

using a couple of external plugins for checking for build failures we introduce unwittingly. I was nervous in the past about relying on codebases we don't control for our own testing, but perhaps we should give it a go for now and see how it works over time.

Suggest to start with zeek-more-hashes (due to OpaqueVal usage and under our control) and the zeek-kafka plugin due to being popular and relatively stable, plus having third-party library dependencies.

Testing should be done using the Zeek installation tree on Linux and OSX (the latter being interesting due to some non-standard include paths for OpenSSL and other dependencies).

Alternatively, dumping a baseline of public zeek:: function signatures and classes with their public (and protected if class not final) methods could also provide some signal that we're causing an API break unwittingly.

Possibly https://lvc.github.io/abi-compliance-checker/ (used a long time ago by @bbannier)