Allow FQCN in value attribute of discriminator-mapping field

[MatteoFeltrin]

Bug Report

We are storing to database FQCN to later use them to know which class the Entity has to be casted to through a Discriminator map.
We found out the problem updating to newer version of Doctrine which changed the default value of $isXsdValidationEnabled to true.
Actually, the attribute value of discriminator-mapping is set as XS:NMTOKEN in the XSD schema

Our XML Configuration of the entity:

<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping https://www.doctrine-project.org/schemas/orm/doctrine-mapping.xsd">
  <entity name="App\Core\EntityVersioning\Domain\Model\EntityVersion" table="entity_version" inheritance-type="SINGLE_TABLE">
    <discriminator-column name="entityType" type="string"/>
    <discriminator-map>
      <discriminator-mapping value="App\Core\Part\Domain\Model\Part" class="App\Core\Part\Domain\Model\PartVersion" />
      <discriminator-mapping value="App\Core\Card\Domain\Model\Card" class="App\Core\Card\Domain\Model\CardVersion" />
    </discriminator-map>
    <field name="version" type="string" nullable="false"/>
    <id name="entityId" type="string"/>
    <id name="entityType" association-key="true"/>
  </entity>
</doctrine-mapping>

Found out this related issue:
#10627

Q	A
BC Break	no
Version	3.1.x

Summary

Allow FQCN in value attribute of discriminator-mapping (or just loosen the type validation)

Current behavior

An exception is thrown when the XML is validated against the XSD configuration.

        Doctrine\ORM\Mapping\MappingException: libxml error: Element '{http://doctrine-project.org/schemas/orm/doctrine-mapping}discriminator-mapping', attribute 'value': 'App\Core\Part\Domain\Model\Part' is not a valid value of the atomic type 'xs:NMTOKEN'.
        in /src/Common/Infrastructure/Persistence/Doctrine/XmlMetadata/App.Core.EntityVersioning.Domain.Model.EntityVersion.dcm.xml at line 6
        libxml error: Element '{http://doctrine-project.org/schemas/orm/doctrine-mapping}discriminator-mapping', attribute 'value': 'App\Core\Card\Domain\Model\Card' is not a valid value of the atomic type 'xs:NMTOKEN'.
        in /src/Common/Infrastructure/Persistence/Doctrine/XmlMetadata/App.Core.EntityVersioning.Domain.Model.EntityVersion.dcm.xml at line 7

How to reproduce

Just write a backslash, or FQCN, inside the attribute "value" of a discriminator mapping field and let it be validated against XSD schema

Expected behavior

No exception to be thrown