Publish fdb_c headers as a downloadable build artifact

For .deb and .rpm client packages, we install both libfdb_c.so and the `foundationdb/*.h` headers necessary to build an application and link against it. For platforms other than debian/ubuntu and fedora/centos, we offer tarballs of the binaries for download, but no headers. If one wants the headers, they'd currently have to download and unpack a .dep or .rpm.

We should separately publish the headers so that they're still accessible for people not on a platform that we already have packaging support for.