lib: extract unixfs filestore into lib

[nonsense]

Proposed Changes

This PR is extracting and exporting the function which creates a UnixFS Filestore - I'd like to be able to use it from other packages, and right now it is private.

Checklist

Before you mark the PR ready for review, please make sure that:

• All commits have a clear commit message.
• The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• This PR has tests for new functionality or change in behaviour
• If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• CI is green

[codecov]

Codecov Report

Merging #8354 (0ed8647) into master (a94e47c) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8354      +/-   ##
==========================================
- Coverage   40.23%   40.18%   -0.05%     
==========================================
  Files         682      682              
  Lines       74428    74428              
==========================================
- Hits        29946    29911      -35     
- Misses      39247    39273      +26     
- Partials     5235     5244       +9     

Impacted Files	Coverage Δ
lib/unixfs/filestore.go	49.29% <100.00%> (ø)
node/impl/client/client.go	51.37% <100.00%> (ø)
storage/wdpost_sched.go	75.49% <0.00%> (-5.89%)	⬇️
extern/storage-sealing/currentdealinfo.go	71.17% <0.00%> (-3.61%)	⬇️
node/hello/hello.go	63.63% <0.00%> (-3.41%)	⬇️
chain/vm/mkactor.go	48.48% <0.00%> (-3.04%)	⬇️
chain/sub/incoming.go	45.25% <0.00%> (-2.45%)	⬇️
blockstore/autobatch.go	56.30% <0.00%> (-1.69%)	⬇️
extern/sector-storage/sched.go	83.12% <0.00%> (-1.65%)	⬇️
chain/events/events_called.go	83.90% <0.00%> (-1.47%)	⬇️
... and 10 more