feat: OptionalString and UnixFSShardingSizeThreshold

[aschmahmann]

Also comes with a type definition for optional strings.

[lidel]

OptionalString lgtm.

To avoid writing size parsers in multiple places, we may want to add Size type similar to Duration that takes care of serializing and deserializing bytes to human-readable form like "10GB", but this could be added in the future if we run out of time this week.

[aschmahmann]

We can use go-humanize for the conversion since we already use it for this job in go-ipfs. e.g. https://github.com/ipfs/go-ipfs/blob/ef866a1400b3b2861e5e8b6cc9edc8633b890a0a/core/corerepo/stat.go#L81

[lidel]

Tested with ipfs/kubo#8527 and works as expected.

Note to self: we need OptionalString in other places, so let's land this as-is and add dedicated type for humanized byte sizes in a separate PR.

[lidel]

Merging to unblock #150 which in turn will unblock sharness tests in libp2p update