persisters/filesystem_cached: Add file-based persister with cache

[gioele]

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This persister keeps a cache of cassettes, to avoid the overhead of constantly serializing and deserializing unchanged cassettes.

This is especially useful when no changes to the cassette are expected, for example when running testsuites with mode=none.

In a testsuite with hundreds of calls backed from YAML cassettes with 10k responses, the use of this persister saves various minutes.

[codecov-io]

Codecov Report

Merging #576 (d6ed6e0) into master (6e04003) will increase coverage by 1.05%. The diff coverage is 78.66%.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
+ Coverage   89.01%   90.06%   +1.05%     
==========================================
  Files          24       27       +3     
  Lines        1539     1742     +203     
  Branches      212      243      +31     
==========================================
+ Hits         1370     1569     +199     
+ Misses        139      135       -4     
- Partials       30       38       +8     

Impacted Files	Coverage Δ
vcr/persisters/filesystem_cached.py	0.00% <0.00%> (ø)
vcr/stubs/aiohttp_stubs.py	94.30% <88.05%> (ø)
vcr/stubs/httpx_stubs.py	97.80% <97.80%> (ø)
vcr/cassette.py	97.15% <100.00%> (+0.98%)	:arrow_up:
vcr/config.py	95.00% <100.00%> (+0.03%)	:arrow_up:
vcr/patch.py	89.41% <100.00%> (+5.04%)	:arrow_up:
vcr/record_mode.py	100.00% <100.00%> (ø)
vcr/serializers/jsonserializer.py	100.00% <100.00%> (ø)
vcr/serializers/yamlserializer.py	77.77% <0.00%> (-22.23%)	:arrow_down:
vcr/migration.py	74.48% <0.00%> (-2.05%)	:arrow_down:
... and 7 more

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