Add Helm chart for Kubernetes deployment

### Prerequisites

- [x] Search the [current open issues](https://github.com/googleapis/genai-toolbox/issues)

### What are you trying to do that currently feels hard or impossible?

👋

Based on the [existing documentation](https://googleapis.github.io/genai-toolbox/how-to/deploy_gke/), we can create a Helm chart for making deployments of the MCP toolbox easier & standardized on Kubernetes.

As an example of a potential benefit, we could have `values` options for # of replicas (servers), horizontal auto-scaling (HPA), explicit resources, etc. for increased reliability.

### Suggested Solution(s)

- `helm/genai-toolbox/` or `helm-charts/` as a new top-level directory, containing:
- `Deployment` - for the MCP server(s)
- `topologySpreadConstraints` & [anti-]affinities to run instances across AWS AZs / GCP Zones.
- `Service` - for access
- `HorizontalPodAutoscaler` - for the auto-scaling option
- `ServiceAccount` - can bring-your-own or option to define it in chart
- `Secret` - for `tools.yaml`
- ...

### Alternatives Considered

_No response_

### Additional Details

I'd be interested in contributing this; wanted to raise it for feedback.