Add simple struct for praefect startup checks

This MR provides a simple framework package for creating praefect startup checks to check the health of the cluster setup.

This MR also includes changes to add a migrations check, and to add a subcommand to expose this check.

closes #3846 (closed)

Note: this is the most basic iteration. In the future we can add more features to this subcommand such as the ability to specify a certain check etc.