fix(blockstore-fs): allow non sha-256 multihashes

[dozyio]

Multihashes aren't CID's thus decode fails when not using a sha-256 mutlihash.
It currently works fine with sha-256 (code 18) multihashes as 18 is the prefix to CIDv0 thus the decode works ok.

The CID.decode here https://github.com/ipfs/js-stores/blob/main/packages/blockstore-fs/src/sharding.ts#L64 eventually calls inspectBytes https://github.com/multiformats/js-multiformats/blob/master/src/cid.ts#L345 which fails.

Proposed fix is to use CID.bytes instead of CID.multihash.bytes.

Potentially a breaking change as any current files won't be accessible.

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