Add toolchains for plugins

Related:
- #41273
- #38078

To finally rid myself of building Protobuf from source, I also need the option to use pre-built gRPC plugins, which normally require building most of Protobuf from source as well. I filed #38078 a while ago to ask for official binary releases, but first we need the option to bring our own builds if we choose to. That means introducing Bazel toolchain types for each plugin.

Once the toolchains exist, we can use a solution similar to #41273 to simply use the toolchains if a flag is enabled, and fall back to building from source if not. Users would have to source binaries for those toolchains themselves until #38078 is closed (if ever), but that's not too hard to work around.