Feat: support for multiformat represenation in IPLD schema

Today there is no good way describe multiformats like https://github.com/multiformats/multihash the best we can do is basically something along the following lines:

```ipldsh
type SignedMessage struct {
payload: Bytes
--
sig: Bytes
}
```

It would be a lot nicer if we could actually define the structure for `sig` field there instead of some comment e.g.

```ipldsch
type SignedMessage struct {
payload: Bytes
sig: Signature
}

type Signature struct {
alg: Integer
size: Integer
out: Bytes
} representation multiformat {
order ["alg", "size", "digest"]
}
```

This in practice could be something between the lines of [tuple](https://ipld.io/docs/schemas/features/representation-strategies/#struct-tuple-representation) and [byteprefix](https://ipld.io/docs/schemas/features/representation-strategies/#union-bytesprefix-representation) representations where:

1. All fields MUST have either `Bytes` or `Integer` representation.
2. `Integers`'s are encoded as varints.
3. `Bytes` are just appended.

Not only this would help us have better schemas when multiformats are involved, but also provide a better language for describing multiformats than a markdown files with some prose.