[csharp-netcore] Fix incorrect FileParameter deserialization

[NoTuxNoBux]

Fixes #10588.

OpenAPI responses with binary content have a method generated that returns a FileParameter with the csharp-netcore generator, but this type can't actually be deserialized properly when the call is made, resulting in an error. The type is simply a wrapper for a stream, so reading the stream and creating the FileParameter is sufficient to fix the problem.

Testing done is that we've tried this fix proposed in #10588 successfully and are using it in production with an OpenAPI that has the following 200 response definition for a GET request that downloads a file:

{
    "description": "The call was successful.",
    "content": {
        "image/png": {
            "schema": { "type": "string", "format": "binary" }
        }
    }
}

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[NoTuxNoBux]

Looks like CI failed with a temporary connection error, I'll force-push a new commit to retrigger it.

[wing328]

LGTM. Thanks for the fix.