Feature: S3 method to enable public access

[DavidChristiansen]

Request the following feature to enable access by 'http://acs.amazonaws.com/groups/global/AllUsers'
to a given s3 bucket.

const bucket = new s3.Bucket(this, 'cdk-lab');
bucket.grantPublicAccess();   

The same can be achieved through the following code snippet

bucket.addToResourcePolicy(new cdk.PolicyStatement()
            .addAction('s3:GetObject')
            .addResource(bucket.arnForObjects('*'))
            .addPrincipal(new cdk.Anyone()));