Introduce siteKey argument for guillotine

[rymsha]

New optional argument siteKey on guillotine GraphQL endpoint.
It defaults to x-guillotine-sitekey header value. (note, that in G8 we remove this header entirely)

resolvers of guillotine children have access to siteKey argument via localContext.
if siteKey is not set (even via x-guillotine-sitekey), the localContext value to start with is '/' (aka project) and it is up to the developer to choose how to deal with.