Add ETag Support to .NET rewrite

[Kejax]

Add support for ETags, which provide a way for applications to verify if a given endpoint still returns the last cached data. This reduces load and bandwidth

With every fetch from the AH-API, the server could generate a hash like MD5 and response with the ETag Header which holds the value of the hash.
A client can take this value and use it in the If-None-Match header. If the hash is still the same, because AH-API data is still the same or the next request from the client has been done before a new fetch occured, the Server answers with a 304 - Not Modified response without body, and with the normal response body, if the data has been changed.

This reduces the bandwidth needed and would be a nice addition, please ask if I explained something bad or if there still are questions

[Kejax]

How the ETag value is generated is obviously not fixed