ds schema: Rename strategy attribute of field-reference to change-option
ds schema: Rename strategy attribute of field-reference to change-option

This is to avoid confusion in the spec where we currently use the term
strategy to distinguish between event strategy, lookup strategy and now
field strategy.

osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi