frr: add optional grpc support via grpcSupport parameter

[tylerjl]

I've been running FRR with gRPC support for a while now with these changes to the upstream package in an overlay. Thought it'd be worth upstreaming. Notes:

• I'm not thrilled about adding CXXFLAGS to the derivation since it changes the package regardless of whether or not you enable gRPC, but it seems to behave fine. And if upstream isn't setting something like -std explicitly, it makes me think that adding it to the downstream nix package will probably just make it behave a little more like upstream intends.
• The parameter is set to true by default, but I'm happy to set this to false if pulling in grpc and protobuf is a little much.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[thillux]

LGTM

[fpletz]

Is which really a runtime dependency? If it's only used at buildtime, this should go into nativeBuildInputs.