"cdk diff" should take assets into account

When running cdk diff, the toolkit should take assets into account. At the moment, if only assets change, “cdk diff” will show no differences, but “cdk deploy” will update the stack since the asset’s md5 changes.

[brad-jones]

For a quick and dirty hack I just inject a hash of the asset into the stack somewhere, in the case of a lambda just set an environment variable. Then when running cdk diff it’s obvious what will update.

new lambda.Function(this, 'SomeLambda', {
	code: lambda.Code.asset('/path/to/zip'),
	environment: {
		HASH: HashFuncOfYourChoice('/path/to/zip'),
	},
	etc...
});

[eladb]

will be fixed by #2821