feat: add support for env_token_override

[netomi]

This fixes #392 .

You can specify now a new configuration value, e.g. in .otterdog-defaults.json:

{
  "env_token_override": "GH_TOKEN",
}

so when running an operation that only needs a token, the one from the env variable as specified in the override will be used instead.

[AlexanderLanin]

Isn't this a very very specific way to a more generic "fallback provider" solution? Like specify pass and env provider, and if pass is not available fallback to env. Or a provider_selection_override param/env-var?

[netomi]

ok let me rethink that, maybe we can utilize the env provider for that.