Introduce configuration for subo to set defaults

[javorszky]

Right now subo has a bunch of defaults, for example rust as a language for a runnable.

As a developer working at an organization where our main language is not rust, every time I use subo create runnable, I need to pass the --lang argument to set it to my org's language.

It would be neat if we could have a bunch of env vars or a config yaml file where we could set defaults for ourselves.

Acceptance criteria:

• enumerate all the defaults
• all defaults have corresponding env var to set it to something different
• there is a configuration yaml file that subo can read and apply. Env vars override values in the config file
• the subo cli help will list what the default is, and why that is (for example "--lang (default swift, env var)" or "--lang (default go, config file)"
• config file location can be customized using an env var too

[hola-soy-milk]

Just started wondering about this myself with the --native flag.

What if we have a config file to configure whether we want to have runnables/projects be built natively by default?

I'm in favour of this!