feat: add streaming multipart file-server playground

[fpawel]

Purpose

Experimental integration playground for go-openapi/runtime#507.

This manually adapts the generated file-server binder to demonstrate the
intended future go-swagger code generation and handler ownership model.

It is not intended for merge until the runtime API and code generation approach
are settled.

What it demonstrates

• the handler receives a streamed file before the client completes the multipart request body;
• file payloads are read directly from the request body;
• the binder transfers ownership of MultipartFormStream to the handler;
• the handler explicitly chooses between Drain() and Close();
• an external http.MaxBytesHandler limit propagates through the streamed reader and results in HTTP 413.

Black-box test

The integration test uses a real httptest.Server and an io.Pipe request body.

The client sends the multipart headers and the first file bytes, then pauses.
The test verifies that the generated router, binder and handler have already
received and started reading the file before the client resumes and finishes
the multipart body.

The generated binder is currently adapted manually to represent the expected
future code generation.

[fredbi]

Yep that’s the right shape as far as i can tell.
This PR may move to a distinct version of the example (or perhaps two operations showing the 2 operating modes not sure yet) after we are able to generate the proper bindings from go-swagger.

Prototype is convincing.

[fredbi]

I think you made it through with the full round-trip: openapi lib -> go swagger -> examples

excellent feature imho. Thanks a lot @fpawel