fix: relax unixfs blockstore input type

[achingbrain]

We only use the get/put/has methods from the blockstore interface so pick those methods to allow easy use from other codebases.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works