add support for “sidecar” frontmatter at the destination url of proxied resources

with v3, was running into similar needs to #1110 (specifically giving a different title to a proxied page), and had the thought that I could use "sidecar" frontmatter to accomplish the task.

That turned out not to be the case, but the patch to get it working is relatively small. While I would agree this isn't the best solution to the issue (especially with the more unified interface for resource metadata coming in v4) -- it provides a need little fix for some cases in v3.