make secrets fields referenceable to be handle by vault secret management

[seboudry]

This PR add support for Kong Vault secrets as environment variables.
https://docs.konghq.com/gateway/latest/kong-enterprise/secrets-management/

Example usage on Kong deployed with Helm chart.

Helm chart values:

customEnv:
  oidc_quicksign_sso_client_id: "client_id"
  oidc_quicksign_sso_client_secret: "client_secret"

Kong plugin

apiVersion: configuration.konghq.com/v1
kind: KongClusterPlugin
metadata:
  name: oidc-sso
plugin: oidc
config:
  client_id: "{vault://env/oidc_quicksign_sso_client_id}"
  client_secret: "{vault://env/oidc_quicksign_sso_client_secret}"
  realm: master
  discovery: https://mydomain/auth/realms/master/.well-known/openid-configuration

[cristichiru]

Sorry for the late response, can you please update the rockspec? saw just now you made almost the same changes as a previous PR