Collections are not supported for custom encoders

Currently custom encoded types (i.e. decimal and IdType) are not supported for fields with cardinality greater than one.
Attempts to encode/decode such type produce an exception "Collections are not supported for custom encoders".

Some of these include:

  "Parameters",
  // Collections are not supported for custom encoders for: condition-> RuntimeIdDatatypeDefinition[id, IdType]
  "Task",
  // Collections are not supported for custom encoders for: condition-> RuntimeIdDatatypeDefinition[id, IdType]
  "StructureDefinition",
  // Collections are not supported for custom encoders for: condition-> RuntimeIdDatatypeDefinition[id, IdType]
  "MolecularSequence",
  // Collections are not supported for custom encoders for: precision-> RuntimePrimitiveDatatypeDefinition[decimal, DecimalType]
  "StructureMap",
  // Collections are not supported for custom encoders for: condition-> RuntimeIdDatatypeDefinition[id, IdType]
  "PlanDefinition"
  // Collections are not supported for custom encoders for: goalId-> RuntimeIdDatatypeDefinition[id, IdType]

Start by reproducing the encoding and decoding failures for collection-valued custom types listed in the issue, including IdType and DecimalType. Trace the custom encoder path to identify the source of the exception, then verify that the affected examples no longer fail for fields with cardinality greater than one.