[Feature Request] Polycubed HTTP responses should follow the requests "Accept" header

[michelsciortino]

Currently, the Accept header of the clients HTTP requests is completely ignored by Polycubed and its responses always have the ContentType header set as application/yang.data+json.

Polycubed HTTP responses should follow the requests "Accept" header, providing data if capable to return them in the required content type.

I'd like to receive HTTP responses with different content types rather then json.

Use case: if a cube is capable to export metrics in OpenMetrics format, it should be able to response with a text/plain content type.