Allow Partial page render to work with PreviewURLChannelInfo_url mode

ATM it is not possible to work with Partial Page Render (PPR) and org.hippoecm.hst.configuration.channel.PreviewURLChannelInfo_url preview mode enabled.

For Preview integration with applications that rely on PPR, a change is required in line:

https://code.onehippo.org/cms-enterprise/brxm/blob/271e5343ae638b34dc9602bfa3ea64bc18a9275b/community/site-toolkit/components/core/src/main/java/org/hippoecm/hst/container/HstDelegateeFilterBean.java#L461

this part of the code is redirecting PPR requests to the delivery instead of letting them through to the CMS.

I have tested this by adding the following condition to the code:

&& !req.getRequestURI().contains("./"))