UnmarshalJSON is limited to the default Tree

[Wondertan]

Problem

When we deserialize the EDS we hardcode the default tree. This is problematic because computed Row and Col roots of the deserialized eds will be utterly different if another custom tree is used.

Solutions

1. Have a global tree constructor var/registry that UnmarshalJSON would use instead
   • Users would then register their trees by name
     • The name would be serialized in the json and deserialization would then get the proper tree by the name
   • This is exactly what happens with the codec.
2. UnmarshalEDS func that accepts the tree.
   • This works in theory, but https://github.com/filecoin-project/go-jsonrpc/blob/17a35772b24f59ceee25cc8d8177a2b255a659ac/client.go#L614 blocks it
   • If we contribute an option that custom Unmarshall, then we would be able to inject the one for EDS

[Wondertan]

The first solution is bulletproof and requires minimum work outside of rsmt2d, as well the same approach is used for Codec, so we should go with it, imo