common/Formatter: introduce helpers for formatting container types.

common/Formatter: introduce helpers for formatting container types.

Specifically:

- with_array_sections() creates a Formmater::ArraySection, then
  formats every element in the container using the provided
  callable.
  Special handling for std::map: the callable is called with
  both the key and the value.

- with_obj_array_section() dumps containers of objects:
  First - an ArraySection is created; then - for each object
  in the container, the object is formatted within an ObjectSection,
  using the provided callable.
  If the container is an std::map, the callable is called with both
  the key and the object itself.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>