Made Dataset and LRUCache objects pickleable

[AbhinavTuli]

The code block below now works:

ds = Dataset("path/to/dataset")
pickled_ds = pickle.dumps(ds)
unpickled_ds = pickle.loads(pickled_ds)

[codecov]

Codecov Report

Merging #1049 (862a12b) into main (e2be96b) will increase coverage by 0.22%.
The diff coverage is 98.90%.

@@            Coverage Diff             @@
##             main    #1049      +/-   ##
==========================================
+ Coverage   89.76%   89.98%   +0.22%     
==========================================
  Files          94       95       +1     
  Lines        4190     4244      +54     
==========================================
+ Hits         3761     3819      +58     
+ Misses        429      425       -4     

Impacted Files	Coverage Δ
hub/core/storage/s3.py	87.78% <0.00%> (+1.32%)	⬆️
hub/api/dataset.py	94.11% <100.00%> (+1.70%)	⬆️
hub/api/tests/test_api.py	99.16% <100.00%> (-0.01%)	⬇️
hub/api/tests/test_pickle.py	100.00% <100.00%> (ø)
hub/core/storage/lru_cache.py	96.12% <100.00%> (+0.39%)	⬆️
hub/core/storage/memory.py	100.00% <100.00%> (ø)
hub/core/storage/provider.py	98.03% <100.00%> (+0.03%)	⬆️
hub/core/storage/tests/test_storage_provider.py	100.00% <100.00%> (ø)
hub/tests/storage_fixtures.py	100.00% <100.00%> (ø)
hub/util/exceptions.py	76.88% <100.00%> (+0.33%)	⬆️
... and 2 more

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[verbose-void]

re-base and add a new fixture