Add ASDF .tool-versions file

What does this MR do?

It adds a local .tool-versions ASDF configuration file.

Why was this MR needed?

So that asdf install knows which golang version to install for the current branch

What's the best way to test this MR?

Run asdf current golang.

The result should point to the version inside .tool-versions and make reference to the file path on your disk:

$ asdf current golang
golang          1.13.8          /Users/pedropombeiro/src/gitlab.com/gitlab-org/gitlab-runner/.tool-versions

What are the relevant issue numbers?

Closes #27962 (closed)