[BUG] Have a public ECR repo from common images generated in walkthrough

**Describe the bug**
Today for most of the walkthrough we need to generate the app or other images locally to test them out. Images such as grpc_cli takes longer time to generate and is frustrating.

As noted in the closed PR it took 10+ minutes to get the created https://github.com/aws/aws-app-mesh-examples/pull/476

To improve the experience of users running these walkthrough we can give option to pull them simply from a public ECR repos.

**Platform**
EKS, ECS, EC2, etc.

**To Reproduce**
Try step 2 in this [walkthroughs/howto-grpc-ingress-gateway/README.md](https://github.com/aws/aws-app-mesh-examples/blob/main/walkthroughs/howto-grpc-ingress-gateway/README.md#grpc-routing-through-ingress-gateway)

**Expected behavior**
Option to just pull the image from public ECR.

Start with step 2 of walkthroughs/howto-grpc-ingress-gateway/README.md and review the context from closed PR #476. Define how users can pull the walkthrough images from a public ECR repository instead of generating them locally. Done means the walkthrough offers a working public ECR image option while retaining enough guidance to test the gRPC ingress gateway.

Written by the indexing model from the issue text.