Add C++20 modules support

Hi,
I'm not a user of google-cloud-cpp, but I'm tracking the progress of c++20 modules support on https://arewemodulesyet.org/ and this is the issue we are linking to in our table of c++20 modules support.

**What component of `google-cloud-cpp` is this feature request for?** For
example, is this related to bigtable (i.e., something in
`google/cloud/bigtable`), or GCS (i.e., something in `google/cloud/storage`)?

Component: Build system

**Is your feature request related to a problem? Please describe.** A clear and
concise description of what the problem is. Ex. I'm always frustrated when

Faster builds

**Describe the solution you'd like** A clear and concise description of what you
want to happen.

Modules are part of C++20 and should greatly increase compile times, see https://en.cppreference.com/w/cpp/language/modules