Create constants for AWS service principals

[jen20]

Since the function aws.iam.assumeRolePolicyForPrincipal was introduced, code such as this has been common:

const assumePolicy = aws.iam.assumeRolePolicyForPrincipal({
    Service: "vpc-flow-logs.amazonaws.com"
})

Personally I can only remember about three of the commonly used service principal addresses, but this is something where autocomplete can help out! If we export constants of the Principal structure for each service from the aws.iam namespace, this code could be rewritten:

const assumePolicy = aws.iam.assumeRolePolicyForPrincipal(aws.iam.VpcFlowLogsPrincpal)

This has the secondary benefit of being a one-liner, making it less obtrusive to declare directly in the input properties of the resource using the policy, reducing the number of names in scope.

Something to investigate is whether the constants might be better inside the aws.iam package or inside the package for each service with a common name (e.g. aws.ec2.ServicePrincipal, aws.lambda.ServicePrincipal). I am tending towards the former since some services have many principals available (EC2 being a good example), and if several are imported for use without the qualifying namespace they will not conflict.