lxd/network: Adds DHCP range parsing functions
lxd/network: Adds DHCP range parsing functions

These have been moved from lxd bridged device code as now we have a network type it makes sense to move its config parsing functions to its own type.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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