Better support for running workflows in a matrix across multiple SDK versions

[evanweible-wf]

It'd be nice to be able to use gha-dart-oss to test a package on multiple versions of the Dart SDK. For example, 2.19.6 and stable, or stable and beta. This is possible today, but one potential issue is that the checks workflow will always run the formatting check, and formatting output can differ between Dark SDK versions.

Could we support disabling the format check via an input to the checks workflow so that the workflow could be run on multiple SDKs but formatting could be configured to only run on a specific version?

[matthewnitschke-wk]

Works for me!