Allow customizing location of global Serverless config files

[christopher-dG]

Feature Proposal

Allow customizing location of serverless' global config files: ~/.serverlessrc and ~/.serverless.

Description

1. What is the use case that should be solved. The more detail you describe this in the easier it is to understand for us.

Some people don't like having their home directories full of files that they neither use nor want. It's nice to be able to put them in other places, like ~/.config, ~/.local/share, etc. For examples of software that behaves nicely, see Git, GnuPG, htop, Emacs >= 27, Python, Go, ccache, nodejs, jupyter, julia, ruby, mix, asdf, etc.

1. Optional: If there is additional config how would it look

Either automatically search common places with something like appdirs, or allow environment variables that specify the locations of the serverlessrc and serverless dirs. Most tools have priority order: environment -> config dir -> home dir

Similar or dependent issues:

• serverless/serverless#7651