Add C++20 modules support

[kelteseth]

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

[coryan]

Cool stuff. Based on your table this is blocked (or at least made more difficult) by:

• TODO: put Abseil link here
• Support using c++ standard library module rather then #include protocolbuffers/protobuf#14109
• Add C++20 modules support grpc/grpc#36497

In addition:

• This project needs to support C++17 until at least 2027,
• Needs to support Bazel (see Support C++20 modules bazelbuild/bazel#4005)

Any modules support will need to live side-by-side with the includes until those problems are fixed.