Add PodDisruptionBudget to appmesh-gateway helm chart

**Is your feature request related to a problem?**
The appmesh-gateway helm chart has a HPA setting but no PodDisruptionBudget. It would be nice to add this to the chart. The appmesh-controller has this ability already: https://github.com/aws/eks-charts/blob/master/stable/appmesh-controller/templates/pdb.yaml

**Is your feature request related to a specific Helm chart, if yes mention name of the chart?**
appmesh-gateway

**Describe the solution you'd like**
I'd like to add a PodDisruptionBudget to the appmesh-gateway chart. I'm happy to create a PR for it.

**Describe alternatives you've considered**
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Inspect the appmesh-gateway Helm chart and compare it with the referenced appmesh-controller/templates/pdb.yaml file. Render or otherwise validate the chart to confirm the PodDisruptionBudget is included for appmesh-gateway and matches the chart's existing HPA-related configuration.

Written by the indexing model from the issue text.