feat: metadata-only / outbound in-mem store

[ramfox]

Rather than adding data to the store when we iroh add, we can remove duplication by only storing the graph metadata, filestore or database location, and block offsets in our store.

This helps us in the DeltaChat iroh-share use case because we know the DeltaChat data already exists on the local device, so we don't need to store it again internally. This will remove duplication and speed up "add" time (since we don't have to write the block data to disk).

This implementation can be ephemeral since in the iroh-share use case the graph/encoded data only needs to exist as long as it takes to transfer the data.