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

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Faster builds

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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
• https://github.com/protocolbuffers/protobuf/issues/14109
• https://github.com/grpc/grpc/issues/36497

In addition:

• This project needs to support C++17 until at least 2027,
• Needs to support Bazel (see https://github.com/bazelbuild/bazel/issues/4005)

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