Fix Env TOKEN_SERVER_ENVOY_API -> USE_TOKEN_SERVER

[mlajkim]

Description

https://github.com/AthenZ/k8s-athenz-sia/issues/80

TODOs

• [x] Actual understanding of the logic
• [x] Choose which name would fit to the feature we provide
  • Attaching tokens in the header is the de facto standard used by Envoy, nginx and other proxies and therefore there is no needed to clearly define what the feature does. We choose to use the USE_TOKEN_SERVER as the token server should attach the token in the header side (world standard)
• [x] Modify TOKEN_SERVER_ENVOY_API -> USE_TOKEN_SERVER
• [x] Double check and mimic https://github.com/AthenZ/k8s-athenz-sia/pull/78/files
• [x] Some comments must be modified
• [x] Add v4.0.0 with other boolean variables follow the prefix USE_
  • https://github.com/AthenZ/k8s-athenz-sia/issues/65