chore: update protos to circa 2020-05-17 (#50)

Use the same commit we are using for the Bazel builds in
google-cloud-cpp. AFAICT, there are no significant changes, but it
is good to keep both build systems in sync.

fix: avoid warnings with newer CMake versions (#48)

I noticed this with CMake 3.17.1, but may have started earlier. Using
`include()` to load a module does not work because the calls to
`find_package_handle_standard_args()` generate a ton of warnings.
Loading the module using `find_package()` works as expected.

feat: bump googleapis to match version in -cpp (#46)

feat: update googleapis proto version to include bigtable backup prot…

…os (#44)

chore: update to protos as-of 2020-03-11 (#42)

chore: update to protos as-of 2020-02-27 (#40)

chore: bump version numbers (#32)

I need to create a v0.4.x release and the version numbers are at v0.3.0

feat: add Cloud Pub/Sub protos (#31)

chore: update googleapis version (#29)

bug: fix build with Ninja (#26)