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.

[LDeakin]

not allowing null for the base data type fill value

I suppose it is a limitation, but I'd note that we don't have any data types that permit a null fill value. Can anyone think of any that might arise?

Also, for a multiply nested optional type like Option<Option<u8>>, you could potentially have the "null" at different levels. An implementation would have to track the "depth" of the fill value and change it each time it goes through the optional codec.

[d-v-b]

Can anyone think of any that might arise?

I think null will be a valid fill value for the JSON data type