Add a script to generate a bundle of plugins for CocoaPods (#872)
Add a script to generate a bundle of plugins for CocoaPods (#872)

Motivation:

https://github.com/grpc/grpc-swift/pull/869 adds a plugin containing the
protoc binaries for CocoaPods. It relies on having a zip of the binaries
in the tagged release.

Modifications:

- Add a script to generate a zip of the protoc binaries.

Result:

We can generate a bundle of plugins which may be pulled in by a
CocoaPod.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift