Add singular resource support to polymorphic_path

[maclover7]

A somewhat less invasive redo to #23535. The problem that makes #1769 so
hard to solve, is that polymorphic_path is only provided with an
database object, so this makes it hard to reinvent the routing tree.
This is the best solution I could come up with.

Will update documentation if this is merged.

r? @pixeltrix

[sgrif]

This is basically the same as #15725, and has the same problems as it. This adds a major slow down to a hotpath, and as we discussed in that PR, we ultimately decided that this was an uncommon case that these methods aren't meant to handle. This is more of a fundamental design problem in the router itself that eventually needs to be addressed. It's better to avoid bandaids like this until then.