Extract Flutter version from asdf

[chris-wickens]

Use case

It would be great if Codemagic could extract the Flutter version from asdf's .tool-versions file, similar to the FVM support in place currently. asdf is a version manager compatible with a variety of technologies, which makes it popular with developers building across multiple stacks. 

https://github.com/oae/asdf-flutter
https://asdf-vm.com/

Idea

Support extracting the Flutter version from the .tool-versions file if one is present, to save having to specify it in both codemagic.yaml and .tool-versions.

[mikhail-tokarev]

Hi Chris,
thank you, this is a good idea! I will keep this issue open to see if it gets traction.
Thanks again

[Jaaco]

This sounds like a really good idea, I would really love this.

[Vitalya1234g]

Thank too