feat: vault client can read a different service account token path

[abrahamjoc]

This PR allows to read a different service account token that can have permissions to auth with vault for getting vault token, but it doesn't have permissions to create a secret in the Kubernetes cluster.

---

Key Changes:

1. Token Path Flexibility: Implementation of functionality that allows the Vault client to read service account tokens from customized paths.
2. Documentation Update: Modifications in charts/README.md to explain how to set up this new option.
3. Code Adjustments: Update in vault/vault.go to support this flexibility in the token path.

---

Benefits:

• Improved security through the use of tokens with specific permissions.
• Increased versatility in the configuration of authentication with Vault.

[ricoberger]

Hi @abrahamjoc, lgtm thanks for your contribution 🙂