Allow to override page-povider headers to narrow fetched properties

There are a number of elements using 'nuxeo-page-provider' including 'nuxeo-search-form' and 'nuxeo-results-view' that configure the header X-NXfetch.document" to 'properties' by default.

Using this header like this has an impact on performance since its resolves all the external entities even if they are not potentially used on the page. https://jira.nuxeo.com/browse/NXP-16066

We have noticed that in a sample app, a call like: POST $server/nuxeo/api/v1/search/pp/asset-search/execute takes about 3s to return 79 docs and only 17ms where spent in ES.

Removing the generic header and passing only a specific property to be resolved improved the execution time 10 times, down to 300ms.
( the app has caches on the directories configured)

For example, for dc:contributor we would fetch automatically all the groups ( as the UserManagerResolver is invoked).

There are two things to fix:

• we should be able to pass a custom header when using the above elements
• the default value should not be 'properties' , probably this will cause performance problem even in the default webUI