We should add support for Referrer-Policy via a header. This would be off by default to remain passive. Examples might be:

<http>
    <headers>
        <!-- defaults to using "no-referrer" -->
        <referrer-policy />
    </headers>
</http>

<http>
    <headers>
        <referrer-policy policy="same-origin"/>
    </headers>
</http>

    http
        .headers()
            // defaults to using "no-referrer"
            .referrerPolicy().and()

    http
        .headers()
            // defaults to using "no-referrer"
            .referrerPolicy(ReferrerPolicy.SAME_ORIGIN).and()

The possible values come from https://www.w3.org/TR/referrer-policy/#referrer-policies

Changes should

• Update the XML and Java Config and include tests
• Update the the rnc files used to create the schema the schema (can be generated from the config directory using ../gradlew rncToXsd
• Update the documentation (both headers section of the docs and the namespace appendix)

NOTE: A good example of the touch points can be found in #3763