Refactor GAPIC's BUILD.bazel files

[dbolduc]

The BUILD.bazel files are fairly cookie cutter. They are generated as scaffolding, then maintained by hand. Making sweeping changes across them (e.g. defining public headers) can be cumbersome. Factoring out the definitions of these components will help reduce future maintenance costs.

---

Any library that is using our CMake helper can probably benefit from this helper. To get a list of libraries, I used:

grep -r "gapic_library" --include "CMakeLists.txt" -l | xargs dirname | sort

On first glance, I only see nuance in:

• bigquery, iam: handwritten samples that need a dep on the mocks.
• securitycenter: Not compatible on Windows, although @devbww tells me it will be compatible when we pick up Protobuf v27