Custom actions when publishing/checking the schema

Instead of building and validating the schema in one go, we could do it in a few steps visible to the user:

1. build
1. validate
1. check with Inspector
1. custom check
1. another custom check

It doesn't have to be done with GraphQL Subscriptions to show the status of each action in real-time. Showing everything at once, completed is good enough. It would open us up for more built-in checks like linting.