[feat] Enable YAML configuration files

[vkarak]

This PR introduces YAML configuration files and deprecates raw JSON configuration files.

Check the updated docs for more details:

• docs/config_reference.rst
• docs/manpage.rst

Closes #3322.

Todos

• Issue deprecation warning when a JSON configuration file is loaded.

[vkarak]

I'm converting this PR to draft, since I had some second thoughts on the sh binding. It's safer if we remove it. The same effect could be achieved through an environment variable, but ReFrame will not be executing arbitrary code:

FOO=$(echo "hello") 

and then using getenv("FOO") in the config.

I'm adding some other useful bindings such as username, groupname, uid, and gid.

[victorusu]

This PR is awesome! lgtm.