Add ability to configure allocation_strategy

[ianks]

This PR adds the ability to configure which instance allocation strategy to use.

Wasmtime::Engine.new(allocation_strategy: :pooling)

allocation_strategy = Wasmtime::PoolingAllocationConfig.new do |c|
  c.total_memories = 42
end
Wasmtime::Engine.new(allocation_strategy:)

[saulecabrera]

I've pushed a commit here #322 which would solve the macos-latest and ruby 3.1 issues, once that's merged, you can rebase.