Add Query support for Nodes (no filtering, no RPC)

This is the initial support for Query2: basic infrastructure (except
filtering) and node query support (without RPC).

It implements all the fields (tests by comparison with list-fields on
the Python side), except that:

- filter is not done
- since RPC is not integrated yet, the runtime gathering/computing is
  simply stubbed out

However, the infrastructure seems pretty reasonable, so I'm sending as
is.

Note that I've split the functions/declarations into multiple files,
to keep each file clean and readable.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>