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GH-44448: [C++] Add support for overwriting grpc_cpp_plugin path for cross-compiling (#44507)

### Rationale for this change

We can't use `find_package(gRPC)` and `gRPC::grpc_cpp_plugin` for cross-compiling because it's for host. We need `grpc_cpp_plugin` for target in cross-compiling.

### What changes are included in this PR?

Add `ARROW_GRPC_CPP_PLUGIN` CMake option that overwrites `gRPC::grpc_cpp_plugin` path found by `find_package(gRPC)`.

### Are these changes tested?

Yes.

https://github.com/conda-forge/arrow-cpp-feedstock/pull/1432

### Are there any user-facing changes?

Yes.
* GitHub Issue: #44448

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>