canonical method for checking locality of a block via coreAPI

#### Location

:wave: 👨‍👧‍👧! I'm looking for the best way to ask "hey is this block local"?

Given that we're in IPFS user land, we should be using core API for this stuff. Which means we're trying to do stuff that's easiest accomplished with a `blockstore` via `coreapi`.

Blockstore method that works _exactly_ as we'd like:

https://github.com/ipfs/go-ipfs-blockstore/blob/3eee0dff760dea94c59381e9a18b9aa290361b28/blockstore.go#L36

Relevant coreAPI subsystem:

https://github.com/ipfs/interface-go-ipfs-core/blob/1c94e6217184ed07abab840143aa1f417e314c3d/block.go#L20-L37

#### Description

We also have the same question about complete DAGs, happy to file a separate issue if you'd prefer. I'd love to be pointed to the canonical methods of determining if a block or dag is local via core API.

Thanks!