Implement default values for flags and fields. (usnistgov#220)

During pairing on usnistgov/OSCAL#1160, we determined there will be a
lot of utility to have default values when enhancing the draft version
of the Rules Model. This PR implements that functionality in the Metaschema
core syntax defined in the XSD.

Co-authored-by: David Waltermire <david.waltermire@nist.gov>

schema/xml/metaschema.xsd

@@ -264,7 +264,7 @@
       <xs:element name="remarks" type="RemarksType" minOccurs="0"/>
       <xs:element name="example" type="ExampleType" minOccurs="0" maxOccurs="unbounded"/>
     </xs:sequence>
-    <xs:attribute name="as-type" type="FieldDatatypesType" default="string"/>
+    <xs:attributeGroup ref="FieldValueAttributeGroup"/>
     <xs:attribute name="collapsible" type="YesNoType" default="no"/>
     <xs:attribute name="name" use="required" type="ModelNameType"/>
     <xs:attribute name="scope" type="ScopeType" default="global"/>
@@ -286,8 +286,7 @@
       <xs:element name="example" type="ExampleType" minOccurs="0" maxOccurs="unbounded"/>
     </xs:sequence>
     <xs:attribute name="name" use="required" type="ModelNameType"/>
-    <!-- datatype should default to 'string' -->
-    <xs:attribute name="as-type" type="SimpleDatatypesType" default="string"/>
+    <xs:attributeGroup ref="FlagValueAttributeGroup"/>
     <xs:attribute name="scope" type="ScopeType" default="global"/>
     <xs:attributeGroup ref="DeprecationAttributeGroup"/>
   </xs:complexType>
@@ -318,7 +317,6 @@
     <xs:attributeGroup ref="DeprecationAttributeGroup"/>
   </xs:complexType>
 
-
   <xs:complexType name="InlineFieldDefinitionType">
     <xs:annotation>
       <xs:documentation>In JSON, an object with a nominal string value (potentially with internal
@@ -339,7 +337,7 @@
       <xs:element name="remarks" type="RemarksType" minOccurs="0"/>
       <xs:element name="example" type="ExampleType" minOccurs="0" maxOccurs="unbounded"/>
     </xs:sequence>
-    <xs:attribute name="as-type" type="FieldDatatypesType" default="string"/>
+    <xs:attributeGroup ref="FieldValueAttributeGroup"/>
     <xs:attribute name="collapsible" type="YesNoType" default="no"/>
     <xs:attributeGroup ref="CardinalitySpecificationGroup"/>
     <xs:attribute name="name" use="required" type="ModelNameType"/>
@@ -363,8 +361,7 @@
       <xs:element name="example" type="ExampleType" minOccurs="0" maxOccurs="unbounded"/>
     </xs:sequence>
     <xs:attribute name="name" use="required" type="ModelNameType"/>
-    <!-- datatype should default to 'string' -->
-    <xs:attribute name="as-type" type="SimpleDatatypesType" default="string"/>
+    <xs:attributeGroup ref="FlagValueAttributeGroup"/>
     <xs:attribute name="required" type="YesNoType" default="no"/>
     <xs:attributeGroup ref="DeprecationAttributeGroup"/>
   </xs:complexType>
@@ -507,6 +504,17 @@
     </xs:attribute>
   </xs:attributeGroup>
 
+  <xs:attributeGroup name="FieldValueAttributeGroup">
+    <xs:attribute name="as-type" type="FieldDatatypesType" default="string"/>
+    <xs:attribute name="default" type="StringDatatype" default="string"/>
+  </xs:attributeGroup>
+
+  <xs:attributeGroup name="FlagValueAttributeGroup">
+    <!-- datatype should default to 'string' -->
+    <xs:attribute name="as-type" type="SimpleDatatypesType" default="string"/>
+    <xs:attribute name="default" type="StringDatatype"/>
+  </xs:attributeGroup>
+
   <xs:simpleType name="NaturalNumberOrUnboundedType">
     <xs:union memberTypes="PositiveIntegerDatatype">
       <xs:simpleType>