Add support for a custom executable php-cs-fixer & docker paths

[amurrell]

I'm posing this PR more as a suggestion or offer - not as final code, necessarily.

I was running into an issue of not wanting to install php locally in order to support php-cs-fixer - but instead run it through my project's corresponding docker container.

With a few more configuration settings, you can offer the option to swap out the php-cs-fixer to another executable (in my case, docker exec... ) and you can ask users for paths to swap out so they can be relative to the container instead of the host machine.

Explained by issue here:

https://stackoverflow.com/questions/67067987/how-to-use-php-cs-fixer-in-vscode-with-docker-container-for-php-and-composer/67067988#67067988