[C++] Use FetchContent for bundled google-cloud-cpp

[raulcd]

Describe the enhancement requested

In order to reduce complexity of our ThirdParty dependency system we would like to remove all abseil target definitions. In order to do so we should move Google Cloud CPP fromExternal Project to FetchContent.

• [C++] Refactor bundled dependencies from ExternalProject to FetchContent #45303

This issue is to track specifically moving google-cloud-cpp.

This follows a similar approach to what a different PR was doing with abseil, c-ares, RE2, Protobuf, gRPC

• [C++] Use FetchContent for bundled Abseil #48074
• [C++] Use FetchContent for bundled c-ares #48091
• [C++] Use FetchContent for bundled RE2 #48178
• [C++] Use FetchContent for bundled Protobuf #48181
• [C++] Use FetchContent for bundled gRPC #48248

Once this is done we should be able to simplify building abseil.

Component(s)

C++