Provide mTLS walkthrough for east-west traffic

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**Describe the bug**
A clear and concise description of what the bug is.

- mTLS walkthrough's provide examples for Gateway to application mtls configuration. However, this is not end to end. Considering a application with multiple backends in same mesh, we need an mtls example to shows following,

```
gateway -> app1 -> app2
```
Note: mTLS enabled between `gateway -> app1`, `app1 -> app2`, `gateway-> app1 -> app2`

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

- EKS, ECS

**To Reproduce**
Steps to reproduce the behavior:
1. Go to [aws-app-mesh-examples](https://github.com/saiteja313/aws-app-mesh-examples/tree/main/walkthroughs)/[howto-mutual-tls-file-provided]
2. Follow instructions and deploy setup.
3. Validate that mTLS is configured from `gateway -> blue (color)` but no mTLS between `blue -> green`

**Additional context**
Add any other context about the problem here.
- We can add a note about this stating, `to enable support for east-west traffic, you can add backend listener pointing on client app pointing to server app`.

Start with the walkthrough at walkthroughs/howto-mutual-tls-file-provided and reproduce its gateway-to-blue setup on EKS or ECS. Extend the walkthrough to cover gateway -> app1 -> app2 with mTLS on both hops, then validate that traffic between the services is encrypted and document the backend-listener configuration needed for east-west traffic.

Written by the indexing model from the issue text.