[Feature Request]: Support Kubernetes authentication for HashiCorp Vault variable resolver

### What would you like to happen?

### Problem

Apache Hop's HashiCorp Vault variable resolver currently supports authentication using long-lived Vault tokens.

While this works, using long-lived tokens is not ideal in Kubernetes environments, where workloads can authenticate using short-lived, automatically managed credentials.

We are currently implementing the HashiCorp Vault variable resolver in a Kubernetes-based environment, and would like to avoid having to provision and manage long-lived Vault tokens for Hop.

### Proposed solution

Add support for the Kubernetes authentication method to the HashiCorp Vault variable resolver.

The resolver should be able to authenticate against Vault using a Kubernetes ServiceAccount token, allowing Vault to validate the workload identity through its Kubernetes authentication backend.

The expected flow would be roughly:

- Apache Hop is running inside a Kubernetes pod.
- The pod has a Kubernetes ServiceAccount assigned to it.
- Hop reads the ServiceAccount JWT/token available inside the pod.
- Hop authenticates against Vault using the Kubernetes auth method.
- Vault returns a short-lived Vault token.
- The resolver uses that token to retrieve the requested secrets.
- The Vault token can be renewed or re-authenticated as appropriate, without requiring a long-lived token to be configured in Hop.
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### Motivation

Using Kubernetes authentication would provide several advantages:

- Avoids storing long-lived Vault tokens in Hop configuration.
- Uses short-lived credentials managed by Kubernetes.
- Better aligns with Vault's recommended authentication mechanisms for workloads running in Kubernetes.
- Reduces the risk associated with leaked or compromised long-lived Vault tokens.
- Makes the HashiCorp Vault variable resolver easier to deploy in cloud-native environments.

### Issue Priority

Priority: 2

### Issue Component

Component: Hop Server

The issue names no files, tests, or resolver entry point. Start by locating Apache Hop's HashiCorp Vault variable resolver and its existing long-lived token authentication path, then compare it with the Kubernetes authentication flow described here. Done means the resolver can authenticate with a Kubernetes ServiceAccount token and retrieve secrets using the resulting short-lived Vault token.

Written by the indexing model from the issue text.