`UnmarshalJSON` is limited to the default Tree

## Problem
When we deserialize the EDS [we hardcode the default tree](https://github.com/celestiaorg/rsmt2d/blob/7a8fd8372e84ed73c2e42821b9bb2d0913d18bd1/extendeddatasquare.go#L40C88-L40C88). 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

Start with extendeddatasquare.go and its UnmarshalJSON implementation, then review the linked go-jsonrpc client limitation and the codec behavior mentioned in the issue. Compare the two proposed approaches and confirm that deserializing an EDS built with a custom tree preserves the expected row and column roots; the issue currently leaves the implementation choice open.

Written by the indexing model from the issue text.