How do I add a custom validator to Tavern to validate specific data types?

The Tavern documentation shows how to use an external function to perform custom validation on an entire response:

response:
  status_code: 200
  verify_response_with:
    function: testing_utils:message_says_hello

There are also some built-in type validators to verify that specific fields of a response are of the correct type:

response:
  body:
    date: !anystr

How can I define a custom function that validates a single value within the response header or body?

Some use cases:

• Validate a JSON Web Token that's embedded within a response
• Verify that date strings are of the expected form
• Verify that a field contains a valid email address or phone number