Handle etags for resource fetch

[imsdu]

Motivation
When fetching a resource, we want to return a Etag header to enable conditional caching in order to save bandwidth as Delta won't have to resend the full response.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag

Acceptance criteria

1. When fetch the different kind of resources, a etag header is computed and returned
2. This etag must take into account the representation of the resource (media type, compression)