This is to help simplify the grpc iOS release flow.
Package.swift was manually written and out of sync for a long time in
this repo.
They are currently being updated updated for releases in
[grpc-ios](https://github.com/grpc/grpc-ios) repo, which includes a lot
of manual work to include new files and exclude tests.
This diff generate Package.swift from a template like other languages to
avoid the tedious work. Currently only gRPC-Core is generated form the
template, gRPC-cpp remains the same for now as they don't change often.
CC @sampajano
Hannah Shi