[CMake] Fix `FindGRPC.cmake` for setting up gRPC related libraries for macOS+homebrew context

* Switch `$GRPC_OPTS` references to `${GRPC_OPTS}`
* Use `target_include_directories()` to add include search paths only for the targets the depend on `gRPC`
* Also find and include the search path for `abseil` headers (`gRPC` headers include them)
* Only setup the gRPC related targets once, so that `include(FindGRPC)` can be called from multiple tools

Differential Revision: https://reviews.llvm.org/D127893