cli: support custom config path/file flag instead of relying only on comsiconfig algorithm

[Hotell]

Current Behaviour

user cannot use different static configurations per environment, thus is forced to author beachbal.config.js and use environment variables, because beachball relies on cosmiconfig behaviour.

This leads to workarounds like following:

• https://github.com/microsoft/fluentui/blob/master/.github/workflows/check-packages.yml#L54-L55
• https://github.com/microsoft/fluentui/blob/master/scripts/beachball/getScopes.ts#L13-L15

New Behaviour

beachball CLI provides --config=path/to/my/config.json option to specify custom path from CLI

[Hotell]

after further investigation of source code I can see that this is already implemented

actionable:

I'll send PR with:

• updated docs
• CLI showing proper flags that are supported