Support Receiving Pings for Non Post-Type Permalinks

[dshanske]

Related: #2700

Ten years ago, it was proposed that the ability to ping the homepage(often linked in the author_url of a comment).

After a year, the issue was closed and turned over as a plugin issue. While the idea of pinging comments is certainly one that can be added as a plugin, the infrastructure to support receipt on arbitrary pages is not.

This enhancement is specifically about making it possible for Core or a plugin to receive and direct these pingbacks, separate from the issue of what to do with them once received.

After the changes are made, that could be explored as plugin territory. However, the only way to do this now is to completely replace the pingback handler with a custom one.

Probably the easiest way to do this is to expand url_to_postid to allow it to return an arbitrary post_ID(using a filter) if it detects that the URL in question is a valid URL on the site, but does not refer to a post_type.