Add flag to buildengine WatchEvent to indicate if it was a hash change or schema change

[wesbillman]

This will be useful so we only rebuild dependent modules when a module's schema changes. This will improve performance of the build system.

[wesbillman]

@alecthomas for this one, I think I can watch PullSchema to get schema changes and react accordingly, but it seems like the schema always thinks it changed when we redeploy a module because of this code during the deploy step.

	module.Runtime = &schemapb.ModuleRuntime{
		CreateTime:  timestamppb.Now(),
		Language:    config.Language,
		MinReplicas: replicas,
	}

So when we deploy the CreateTime of the Runtime will be different indicating a change. I could do something like remove that from the schema before comparing, but not sure if that's the best approach or not. Maybe we can sync on this when you have a sec.

Or maybe there's something else I can watch like just the build schema changes vs. deploy changes.

[alecthomas]

Makes sense, you figure something out and send it in the PR.