[repository schema] Support for field presence maps

[mkudukin]

The proposal is to add ability to associate presence map field to the components and groups as discussed in #197.

Issue

Some protocols like HKEX OCG have a feature called Field Presence Maps, which allows to skip transmission of unset optional fields.
It's a special type of field which contains a bitmap describing the presence of subsequent fields in the actual message instance. The presence map provides an efficient way to check which optional fields are present in a received message. Bits corresponding to required fields are always set to 1.

Those types may be represented as special datatypes mapped to bitsets. In HKEX OCG, for example, a field of such datatype is included in a message header and also it precedes every repeating group instance:

<datatype name="Bitmap">
	<mappedDatatype standard="ISO11404" base="bitstring" element="bit">
		<annotation>
			<documentation>Field length is a number of octets.</documentation>
		</annotation>
	</mappedDatatype>
</datatype>

<fields>
	<field id="42" name="BodyFieldsPresenceMap" type="Bitmap" implLength="32">
		<annotation>
			<documentation>Indicates the list of fields that would be present in a message body immediately after this field. Each bit denotes corresponding field's presence.</documentation>
		</annotation>
	</field>
	<field id="101" name="NoEntitlementsBodyFieldPresenceMap" type="Bitmap" implLength="2"/>
</fields>

<groups>
	<group id="100" name="Entitlements">
		<numInGroup id="100" name="NoEntitlements"/>
		<!-- Presence bitmap for this group is the field with id 101 -->
		<fieldRef id="101" name="NoEntitlementsBodyFieldPresenceMap"/>
		<fieldRef id="102" name="EntitlementType"/>
		<fieldRef id="103" name="EntitlementIndicator"/>
		<fieldRef id="104" name="EntitlementID"/>
	</group>
</groups>

<components>
	<component id="1" name="Header">
		<!-- ... -->
		<fieldRef id="42" name="HeaderFieldPresenceMap"/>
	</component>
	<!-- Presence bitmap for this component is the field with id 42 -->
	<component id="10" name="TestRequestBody">
		<fieldRef id="43" name="FirstField"/>
		<fieldRef id="44" name="SecondField"/>
		<groupRef id="100" name="NoEntitlements"/>
	</component>
</components>

<messages>
	<message name="TestRequest">
		<structure>
			<componentRef id="1" name="Header"/>
			<componentRef id="10" name="TestRequestBody"/>
			<componentRef id="2" name="Trailer"/>
		</structure>
	</message>
</messages>

There should be a standard approach to associate the bitmap field with the components and groups it relates to, in the same way a data field may be associated with its length field using the lengthId attribute.

Proposal

We recommend adding optional presenceMapId attribute to container elements like message/structure, component, and group. This attribute must reference a field which contains presence flag for every direct member of the container element. This approach is like the one with the lengthId and discriminatorId attributes of a field definition, which are already a part of the standard.

Example

<components>
	<component id="1" name="Header">
		<fieldRef id="42" name="HeaderFieldPresenceMap"/>
	</component>
	<component id="10" name="TestRequestBody" presenceMapId="42">
		<fieldRef id="43" name="FirstField"/>
		<fieldRef id="44" name="SecondField"/>
		<groupRef id="100" name="NoEntitlements"/>
	</component>
</components>

<groups>
	<group id="100" name="Entitlements" presenceMapId="101">
		<numInGroup id="100" name="NoEntitlements"/>

		<fieldRef id="101" name="NoEntitlementsBodyFieldPresenceMap"/>
		<fieldRef id="102" name="EntitlementType"/>
		<fieldRef id="103" name="EntitlementIndicator"/>
		<fieldRef id="104" name="EntitlementID"/>
	</group>
</groups>