Rethink how GCS+gRPC is built

We are taking a hard dependency on `opentelemetry-cpp` and a soft dependency on gRPC's OTel plugin, even though both are required for metrics. It seems like we would want either both deps to be optional, or neither to be optional.

CMake and Bazel are also out of step.

We need to make progress though, so let's come back to this decision later.

Start by comparing the CMake and Bazel dependency declarations for GCS+gRPC, focusing on opentelemetry-cpp and gRPC's OTel plugin. The issue does not name files or tests, and completion is not defined because the dependency decision was explicitly deferred; a follow-up would need to establish the intended optionality and align both build systems.

索引モデルが issue の本文から書いたものです。