Add support for precompiled gems

This models the configuration in https://github.com/y-crdt/yrb/blob/main/Rakefile.

To test:

bundle exec rake gem:aarch64-linux           
bundle exec rake gem:arm64-darwin       
bundle exec rake gem:x86_64-darwin
bundle exec rake gem:x86_64-linux            
bundle exec rake gem:x86_64-linux-musl

This will create packages in pkg.

To install for Linux x86:

gem install pkg/prometheus-client-mmap-0.20.3-x86_64-linux.gem

To test, run irb:

irb(main):001:0> require 'prometheus/client'
=> true
irb(main):002:0> require 'prometheus/client/formats/text'
=> true
irb(main):003:0> Prometheus::Client::Formats::Text.rust_impl_available?
=> true

Relates to #42 (closed)