Add compatibility with HR Open Standards

Requested https://wordpress.org/support/topic/wp-jobmanager-interacting-and-capable-to-work-with-hr-openstandard/

**HR Open Standards**

> HR Open Standards are available as XML and JSON syntax for describing and exchanging data from human resources. With HR Open Standards, among other things, job advertisements, job descriptions and CVs should be marked in writing.

> HR Open Standards is designed to save employers and intermediaries the risk and expense of ad hoc negotiation of data sharing mechanisms.

> With the development and publication of data exchange standards based on Extensible Markup Language (“XML”), the Consortium provides any company with the means to exchange with other companies without having to establish, design and implement several different exchange mechanisms.

see more:
https://de.wikipedia.org/wiki/HR_Open_Standards

and here: https://www.hropenstandards.org/

Start by reading the linked WordPress support request and the HR Open Standards documentation. Identify which job listings, descriptions, or CV exchanges WP Job Manager would need to support and where those integrations belong. Done should be defined as explicit compatibility requirements and corresponding validation, but the issue does not name files or tests.

Written by the indexing model from the issue text.