Allow SeralizarSpecPlugins to also specify serialization_spec

[pmg103]

Sometimes a plugin may itself need to do some somewhat complicated prefetching which could most easily be expressed as a serialization_spec. This should be an alternative to implementing modify_queryset():

class RemainingCreditCost(SerializationSpecPlugin):
   serialization_spec = [
      {'activity_product_version_items': [
          # etc
      ]}
   ]

    def get_value(self, instance):
        # Use the prefetched values from the spec above (which will NOT appear in the output)

[pmg103]

This would allow a simpler implemenation of the Requires() plugin too:

class Requires(SerializationSpecPlugin):
  def __init__(self, fields):
    self.serialization_spec = fields

Or maybe these should be combined into one