FindContentQuery doesn't use Metric when calculating distance

[morph-dev]

The FindContentQuery defined in crates/portalnet/src/find/iterators/findcontent.rs uses discv5::Key::distance for calculating distance between node and content.

Instead, it should have Metric as a generic type. It should also consider whether it makes sense to use distance or not (e.g. for ephemeral headers and beacon content).