Data format for non-fungible asset data

Currently, data for non-fungible assets is stored as a simple vector of bytes. It could be good to add some structure to this (see https://github.com/0xPolygonMiden/miden-base/pull/2#discussion_r1049525892). The key properties that we'd like to have for NFA data are:

- The data should be serializable into bytes.
- Hash of the data should depend on semantic properties rather than formatting. For example, whitespace or field order changes should not affect data hash.
- The data should be reducible to a display string which reflects internal content. For example, we could iterate over internal fields and show a value for each field.