[grpc] codegen cross compilation support

[JonatannQm]

Is your feature request related to a problem? Please describe.

When I cross compile grpc with codegen feature enabled and attempts to use protobuf_generate I get an issue that my grpc_cpp_plugin is not an executable.
After further inspection it seems that when crosscompiling grpc is not copying the host binaries to the tools folder.
(when comparing it to protobuf's port, it seems like there it is copying the host binaries when cross compiling)

Proposed solution

something along these lines

...
string(COMPARE EQUAL "${TARGET_TRIPLET}" "${HOST_TRIPLET}" gRPC_BUILD_GRPC_PLUGIN_BINARIES)
...
if (gRPC_BUILD_CODEGEN)
    if(gRPC_BUILD_GRPC_PLUGIN_BINARIES)
...
    else()
        file(COPY "${CURRENT_HOST_INSTALLED_DIR}/tools/${PORT}" DESTINATION "${CURRENT_PACKAGES_DIR}/tools")
    endif()
else()
...
endif()

Describe alternatives you've considered

No response

Additional context

No response

[dg0yt]

The host binaries live only in the host install tree, and this is as designed AFAICT.
Iff the vcpkg cmake toolchain file is used, the relevant find_package calls should do the right thing, thanks to wrappers.

[JonatannQm]

@dg0yt
The cmake code I suggested is from the protobuf port see line 75,
where on cross compilation the host protoc is moved to the target tools binary folder.

Maybe im missing something but without this copy the protobuf-generate tries to use the target binary for compilation and fails...

The error I get while cross compiling is while trying to use protobuf-generate after vcpkg installation is done, in that case stuff like CURRENT_HOST_INSTALLED_DIR does not exist anymore