Transport error mode for server HTTP and GRPC APIs

[FZambia]

Proposed changes

Currently Centrifugo server API never returns transport level errors - for example it always returns 200 OK for HTTP API and never returns GRPC transport-level errors.

This PR adds an option to use transport-native error codes instead of Centrifugo error field in response. The motivation is make API friendly to integrate with network ecosystem - for automatic retries, better logging, etc.

In current state PR does not modify Batch and Broadcast APIs which are quite special. For these two calls we can't introduce transport-level errors as the response actually includes results of different independent command execution. Most probably the best we can do is documenting this distinction.

Behaviour may be turned on globally:

• using "api_error_mode": "transport" for HTTP
• or "grpc_api_error_mode": "transport" for GRPC

Also it may be used on per-request basis:

• by setting custom header X-Centrifugo-Error-Mode: transport for HTTP
• Adding custom metadata key x-centrifugo-error-mode: transport for GRPC

Example

Before:

❯ echo '{}' | http POST "http://localhost:8000/api/publish"
HTTP/1.1 200 OK
Content-Length: 46
Content-Type: application/json
Date: Sat, 19 Aug 2023 07:23:40 GMT

{
    "error": {
        "code": 107,
        "message": "bad request"
    }
}

After:

❯ echo '{}' | http POST "http://localhost:8000/api/publish" "X-Centrifugo-Error-Mode: transport"
HTTP/1.1 400 Bad Request
Content-Length: 36
Content-Type: application/json
Date: Sat, 19 Aug 2023 07:23:59 GMT

{
    "code": 107,
    "message": "bad request"
}