C# GenericHost support multiple accept headers and allow access to HttpContentHeaders on response

[EraYaN]

Split 1 of #22062

Expose ContentHeaders on ApiResponse, to access ContentDisposition for example with file downloads.
Very useful for downloading files, you can now access details and many more headers about the actual Content.

Add support for other Accept header values or full arrays when multiple options are present.
This uses the correct (array) for the Accept header for non json responses.

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