feature: allow request to specify subset of fields to serialize

Closes #400 (closed)

The purpose of this issue was to allow the client to request a subset of fields from a container in order to improve performance

Originally we proposed the use of shapes for this, but in fact this MR ignores that in favour of using a list. I found this solution very quickly and it seems to be the minimum to meet our needs. I didn't find a specification to follow for this header, so I created a custom header Accept-Shape for this purpose

@balessan @sylvain you might have comment on how I should change this MR to meet better the standards, as I say this is a quick and functional solution. If we're happy with this MR, we might prefer to rename the header Accept-Model-Fields or something like this, since it's not technically a shape?