Folder metadata in stat cache

[ankitaluthra1]

Description

Adds insertFolder, AddNegativeEntryForFolder and lookup folder in stat cache for HNS buckets.
This PR only adds new methods with corresponding implementations, currently these are not called from any bucket type.

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - NA
2. Unit tests - Done
3. Integration tests - NA

[codecov]

Codecov Report

Attention: Patch coverage is 47.29730% with 39 lines in your changes missing coverage. Please review.

Project coverage is 71.44%. Comparing base (ede5064) to head (bd07899).
Report is 7 commits behind head on master.

Files	Patch %	Lines
...storage/caching/mock_gcscaching/mock_stat_cache.go	0.00%	35 Missing ⚠️
internal/cache/metadata/stat_cache.go	89.74%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2097      +/-   ##
==========================================
- Coverage   72.23%   71.44%   -0.80%     
==========================================
  Files         101      101              
  Lines       10956    12175    +1219     
==========================================
+ Hits         7914     8698     +784     
- Misses       2713     3117     +404     
- Partials      329      360      +31     

Flag	Coverage Δ
unittests	71.44% <47.29%> (-0.80%)	⬇️

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[Tulsishah]

LGTM