Add support for room shares to ShareAPIController

In some cases, the ShareAPIController requires explicit handling of each
type of share (for example, to format a share for a DataResponse). Room
shares are implemented in an external app (Nextcloud Talk), so in order
to keep the controller as isolated as possible from room share specifics
all that explicit handling is done in a helper class provided by the
Talk app.

In other cases it is just enough to call the share manager specifying a
room share type; note that the share manager is guarded against share
types for which there is no provider, so it is not necessary to
explicitly check that before passing room shares to the share manager.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>