Define the paths an IPLD format implementation should support

@Mr0grog makes a good point in https://github.com/ipld/ipld/issues/44#issuecomment-411853095

> I think that, because path definition is a really important part of IPLD, it’s important for each codec to specify the set of paths it supports. We haven’t done a good job of that. For example, ipld/js-ipld-ethereum#25 is *in part* caused by not having a clear definition of what fields/paths an Ethereum resolver must present/resolve (regardless of implementation language, since we want the same path to work in JS and in Go and in Rust and in… etc.).

It would be useful to have a resource that defines the transformations that an IPLD format provider performs to make things path-able, and to know what the available paths are. Should such a resource be an extension of https://github.com/ipld/interface-ipld-format/ ?