Start adding mechanism to allow (and enforce) typed attributes (i.e. BigDecimal required for currency value fields)
big_decimal enforces it being a BigDecimal.
Allow the creation of simple untyped attributes
big_decimal allows multiple attrs to be defined in one call
untyped attribtues allow multiple definitions in one hit
Extract the constrained attr stuff from InitializeAttributes
Bring in the new Attributes stuff to add a BigDecimal unit_amount
migrate to Attributes from InitializeAttributes
remove InitializeAttributes now it's dead code