Add MethodType and MethodRequestBody query parameters

[paskozdilar]

Long story short, please see:

• Discard unused body of Unary and ClientStream methods grpc-ecosystem/grpc-gateway#5331
• v2.26.2 breaks grpc-websocket-proxy grpc-ecosystem/grpc-gateway#5326
• Drain req.body on server stream methods with no body defined grpc-ecosystem/grpc-gateway#5240
• ServerStream client context never closes when using google.protobuf.Empty without body annotation grpc-ecosystem/grpc-gateway#5236

Please see #41 (comment)

[paskozdilar]

Wait, this still doesn't fix the issue from grpc-ecosystem/grpc-gateway#5326...
Please hold until more info.

[paskozdilar]

A few updates:

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The root problem that we're trying to fix here is related to Unary and ServerStream methods (those where client only sends a single request):

• HTTP servers expect client to send body and then close the request
• when wrapping http.Handler in a manner wsproxy does, the request doesn't have to be closed
• grpc-gateway only parses the part of request it is interested in - it doesn't check of excess data or drain the req.Body
• fix (workaround?) was added in v2.26.2 (Drain req.body on server stream methods with no body defined grpc-ecosystem/grpc-gateway#5240) which drained req.Body to prevent context leak
• since wsproxy doesn't close req.Body until the client WebSocket is closed, the req.Body drain blocked indefinitely!

Now, this cannot be solved in grpc-gateway alone. I've proposed a PR with request drain in a new goroutine, but johanbrandhorst was (justifiably) reluctant to add it.

Now, the real issue with WebSocket proxy is that it doesn't close the req.Body in Unary and ServerStream methods.
However, it also has to keep req.Body open for ClientStream and Bidirectional methods.

Additionally, when body is defined in proto, WebSocket must send the request body and then close, and if it is not, it must not send the request body, but close immediately.

So the only two solutions for that were:

1. add codegen from proto to websocket proxy
2. add query parameter for specifying method type and whether body should be sent or not.

This PR implements the solution 2.

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In case this PR is accepted, the documentation should probably be changed to make sure users understand how to use these parameters.

This will also be a backwards-incompatible change, since not specifying methodType will not allow request to pass.
So we should probably raise the major version too?

[paskozdilar]

Actually, if grpc-gateway adds metadata to response header, we could detect the method types dynamically and avoid these params altogether. Maybe even get rid of MethodOverrideParam!

Please hold until grpc-ecosystem/grpc-gateway#5331 is finished.