Perhaps the `TransportInterface` should be moved in a separate package.

[kafkiansky]

Hi.

Currently, there’s no way to create a package with a different implementation of TransportInterface (e.g., a non-blocking one) without requiring the entire package as a dependency just for that single implementation. Perhaps you could move the TransportInterface to a separate package and keep only CurlTransport in this one.

[vjik]

Transport implementation used with base package only. So it's OK when package with transport implementation depends from base package.

I'm planned extract PSR transport in separate package. This will remove all dependencies from base package.