Add state machine principal

[nickroberts]

Hello!

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Issue details

Add states.amazonaws.com service principal to principal constants.

Affected area/feature

Add something like this:

/**
  * Service Principal for Amazon State Machines
  */
export const StateMachineServicePrincipal: Principal = {Service: "states.amazonaws.com"};

to here: https://github.com/pulumi/pulumi-aws/blob/master/sdk/nodejs/iam/principals.ts

[mikhailshilkov]

Thank you for opening this issue @nickroberts! Do you want to try to open a PR for it? It should be fairly straightforward and comparable to #1494 (and no need for manual changelog anymore)