Fix lease duration env var in agent-injector config

[mmcgarr]

The AGENT_INJECT_TEMPLATE_LEASE_RENEWAL_THRESHOLD environment variable on agent-injector pods is not being properly applied to the generated Vault agent configuration on mutated pods.

Expected Behavior: When AGENT_INJECT_TEMPLATE_LEASE_RENEWAL_THRESHOLD is set, the lease_renewal_threshold field should be included in the template_config section of the injected Vault agent configuration.

Current Behavior: The environment variable is ignored, and lease_renewal_threshold is missing from the generated configuration.

{
  "auto_auth": {
    "method": {
      "type": "kubernetes",
      "mount_path": "auth/kubernetes/cluster-01",
      "config": {
        "role": "test_vault-test_vault",
        "token_path": "/var/run/secrets/kubernetes.io/serviceaccount/token"
      }
    },
    "sink": [
      {
        "type": "file",
        "config": {
          "path": "/home/vault/.vault-token"
        }
      }
    ]
  },
  "exit_after_auth": false,
  "pid_file": "/home/vault/.pid",
  "vault": {
    "address": "<redacted>"
  },
  "template": [
    {
      "destination": "/config/mycreds",
      "contents": "{{ with secret \"db/test/creds/readonly\" }}{{ range $k, $v := .Data }}{{ $k }}: {{ $v }}\n{{ end }}{{ end }}",
      "left_delimiter": "{{",
      "right_delimiter": "}}"
    }
  ],
  "template_config": {
    "exit_on_retry_failure": true
  }
}

Expected config:

{
  "auto_auth": {
    "method": {
      "type": "kubernetes",
      "mount_path": "auth/kubernetes/cluster-01",
      "config": {
        "role": "test_vault-test_vault",
        "token_path": "/var/run/secrets/kubernetes.io/serviceaccount/token"
      }
    },
    "sink": [
      {
        "type": "file",
        "config": {
          "path": "/home/vault/.vault-token"
        }
      }
    ]
  },
  "exit_after_auth": false,
  "pid_file": "/home/vault/.pid",
  "vault": {
    "address": "<redacted>"
  },
  "template": [
    {
      "destination": "/config/mycreds",
      "contents": "{{ with secret \"db/test/creds/readonly\" }}{{ range $k, $v := .Data }}{{ $k }}: {{ $v }}\n{{ end }}{{ end }}",
      "left_delimiter": "{{",
      "right_delimiter": "}}"
    }
  ],
  "template_config": {
    "exit_on_retry_failure": true,
    "lease_renewal_threshold": 0.08
  }
}

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[mmcgarr]

Any chance of a review on this? We'd love to get this in if we can 🙏