feature: Add new interface as alternative to existing Marshaler interface

[fxamacker]

Currently, cbor.Marshaler interface matches json.Marshaler to make it easier for users familiar with encoding/json package.

type Marshaler interface {
	MarshalCBOR() ([]byte, error)
}

However, this also inherits the limitations from matching those interfaces, as already noted in comments and discussions at:

• encoding/json/v2 golang/go#63397
• Check well-formedness of data from MarshalCBOR #485 (review) by @benluddy.

For example, the proposed new API for encoding/json/v2 includes MarshalerV2 or MarshalerTo, etc.

Similarly for this library, a new alternative to Marshaler interface can address known limitations of the old API.

[benluddy]

Another wish-list item for this: customized marshal behavior for types implementing a particular interface. That probably implies that you could register multiple custom marshal behaviors matching the same concrete type.

My motivating use case is to interoperate seamlessly with an ecosystem already tied to encoding/json by recognizing types that implement MarshalJSON (but not MarshalCBOR). They'd be marshaled by calling MarshalJSON then transcoding to CBOR, or possibly putting the JSON bytes into a string enclosed by some "encoded JSON" tag.