Make trait for Schema's without being tied to DagCbor

As far as I can tell the only code in this library related to schemas is directly tied to the DagCbor codec. As far as I understand, there is no fundamental reason these things should be tied together as Schemas are related to how the IPLD Data Model.

From the [IPLD Schema Docs](https://ipld.io/docs/schemas/#key-features):

> IPLD Schemas are, of course, based on IPLD. Because we get to reuse the layers of IPLD's [Data Model](https://ipld.io/docs/data-model/) and IPLD's [Codecs](https://ipld.io/docs/codecs/), IPLD Schemas are a design language that *can be used with many different serial formats.*

So disentangling the DagCbor codec from the schema implementations seems somewhat imperative if this library wants to faithfully implement the IPLD design/spec.