Type-based subfields

create.php

<?php
civicrm_api4('MyEntity', 'create', [
  'values' => [
    'severity' => 'error',
    'type' => 'filelog',
    'options' => ['filename' => '/tmp/foo.log']
  ],
]);

foo.xml

<field>
  <name>severity</name>
  <type>string</type>
  <psuedoconstant>
    <options>...info,error,warning...</options>
  <pseudoconstant>
</field>

<field>
  <name>type</name>
  <type>string</type>
  <psuedoconstant>
    <options>...filelog,emaillog,firephplog...</options>
  <pseudoconstant>
</field>

<field>
  <name>options</name>
  <serialize>JSON</serialize>
  <type>text</type>
  <subfields>
    <subfield>
      <matchType>filelog</matchType>
      <name>filename</name>
      <type>string</type>
    </subfield>
    <subfield>
      <matchType>emaillog</matchType>
      <name>email_address</name>
      <type>string</type>
    </subfield>
  </subfields>
</field>