[IMPROVEMENT] add ca_chain support during certificate issuing

Is your feature request related to a problem? Please describe. During certificate issuing Vault returns data.ca_chain field. But PkiResponse ignores it.

see sample response

{
  "lease_id": "pki/issue/test/7ad6cfa5-f04f-c62a-d477-f33210475d05",
  "renewable": false,
  "lease_duration": 21600,
  "data": {
    "certificate": "-----BEGIN CERTIFICATE-----\nMIIDzDCCAragAwIBAgIUOd0ukLcjH43TfTHFG9qE0FtlMVgwCwYJKoZIhvcNAQEL\n...\numkqeYeO30g1uYvDuWLXVA==\n-----END CERTIFICATE-----\n",
    "issuing_ca": "-----BEGIN CERTIFICATE-----\nMIIDUTCCAjmgAwIBAgIJAKM+z4MSfw2mMA0GCSqGSIb3DQEBCwUAMBsxGTAXBgNV\n...\nG/7g4koczXLoUM3OQXd5Aq2cs4SS1vODrYmgbioFsQ3eDHd1fg==\n-----END CERTIFICATE-----\n",
    "ca_chain": [
      "-----BEGIN CERTIFICATE-----\nMIIDUTCCAjmgAwIBAgIJAKM+z4MSfw2mMA0GCSqGSIb3DQEBCwUAMBsxGTAXBgNV\n...\nG/7g4koczXLoUM3OQXd5Aq2cs4SS1vODrYmgbioFsQ3eDHd1fg==\n-----END CERTIFICATE-----\n"
    ],
    "private_key": "-----BEGIN RSA PRIVATE KEY-----\nMIIEowIBAAKCAQEAnVHfwoKsUG1GDVyWB1AFroaKl2ImMBO8EnvGLRrmobIkQvh+\n...\nQN351pgTphi6nlCkGPzkDuwvtxSxiCWXQcaxrHAL7MiJpPzkIBq1\n-----END RSA PRIVATE KEY-----\n",
    "private_key_type": "rsa",
    "serial_number": "39:dd:2e:90:b7:23:1f:8d:d3:7d:31:c5:1b:da:84:d0:5b:65:31:58"
  },
  "warnings": "",
  "auth": null
}

Describe the solution you'd like Add caChain field to io.github.jopenlibs.vault.api.pki.Credential

jopenlibs / vault-java-driver

[IMPROVEMENT] add ca_chain support during certificate issuing #55