feat: optional codec and data type

[LDeakin]

I'm still finalising an implementation, but here is a draft spec.

[jbms]

Looks good, the only issue I see is the fill value representation not allowing null for the base data type fill value, e.g. for a base data type of json or nested optional. Instead you could specify the base data type fill value in a single-element array:

null -> missing
[1] -> value of 1
[null] -> value of null

[mkitti]

While I see the simplicity here of toggling between null and a type, I am wondering if there is a further opportunity to introduce a full type tag to implement tagged unions in Zarr.