Code transforms for HTTP proxy and unidirectional connect

[FZambia]

Proposed changes

Implement code transforms for HTTP proxy and unidirectional connect. Should allow:

• Transformation of http codes to Centrifugo protocol codes (to disconnect code or to protocol error code) in HTTP proxy, as backend can't always control the possible intermediaries in between and sometimes applications already have common middlewares which prevent request to reach business logic handler.
• Centrifugo protocol codes to HTTP codes during connect in unidirectional HTTP-based transports. Which may be convenient for some use cases which want to inherit Centrifugo Uni SSE / Uni HTTP Stream transports but avoid disconnect code specifics upon connection attempt – now for unidirectional connect Centrifugo always returns 200 OK and then sends disconnect message with the reason in case of connection error, then closes connection.

Example of configuration (UPD. was modified taking #906 into account):

{
  "proxy_http_status_code_transforms": [
    {"status_code": 404, "to_disconnect": {"code": 4904, "reason": "not found"}},
    {"status_code": 403, "to_disconnect": {"code": 4903, "reason": "permission denied"}},
    {"status_code": 429, "to_disconnect": {"code": 4429, "reason": "too many requests"}}
  ],
  "uni_sse_connect_code_to_http_response": {
    "enabled": false,
    "transforms": [
      {"code": 4904, "to": {"status_code": 404}},
      {"code": 4903, "to": {"status_code": 403}},
      {"code": 4429, "to": {"status_code": 429}}
    ]
  },
  "uni_http_stream_connect_code_to_http_response": {
    "enabled": false,
    "transforms": [
      {"code": 4904, "to": {"status_code": 404}},
      {"code": 4903, "to": {"status_code": 403}},
      {"code": 4429, "to": {"status_code": 429}}
    ]
  }
}

GRPC proxy is out of scope for now.

This description will be updated.