Negative cache TTL Fix

[ankitaluthra1]

Description

1. This PR adds a new hidden flag for negative cache TTL.
2. The value defaults to 5 secs.
3. The changes include unit tests and are manually verified on GKE as well.
4. Integration tests will be added in subsequent PR.

Link to the issue in case of a bug fix.
NA

Testing details
Manual - Yes
Unit tests - Yes
Integration tests - NA

[codecov]

Codecov Report

Attention: Patch coverage is 45.71429% with 19 lines in your changes missing coverage. Please review.

Project coverage is 75.77%. Comparing base (ff4417a) to head (8d63c9b).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
cfg/config.go	25.00%	4 Missing and 2 partials ⚠️
cfg/validate.go	14.28%	4 Missing and 2 partials ⚠️
cfg/rationalize.go	40.00%	2 Missing and 1 partial ⚠️
internal/gcsx/bucket_manager.go	0.00%	2 Missing ⚠️
cmd/mount.go	0.00%	1 Missing ⚠️
internal/storage/caching/fast_stat_bucket.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2837      +/-   ##
==========================================
- Coverage   75.94%   75.77%   -0.18%     
==========================================
  Files         118      118              
  Lines       16320    16431     +111     
==========================================
+ Hits        12394    12450      +56     
- Misses       3409     3449      +40     
- Partials      517      532      +15     

Flag	Coverage Δ
unittests	75.77% <45.71%> (-0.18%)	⬇️

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[mustvicky]

Nit: files -> files and folders