Add gh actions for erlang/elixir generate testing

[maartenvanvliet]

Adds two actions to test the generation of the sdk's for both elixir and erlang. It's a basic check, just to see whether the code generation will succeed or return an error code.

I've split them up in separate actions as other language specific tests may be added later.

Fixes https://github.com/aws-beam/aws-codegen/issues/42

[jfacorro]

Thank you for the contribution 🎉