feat: added the cacheImmutable option to cache immutable assets (as…

[alexander-akait]

…sets with a hash in file name like image.e12ab567.jpg)

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• documentation update
• typo fix
• metadata update

Motivation / Use-Case

Allow to cache immutable assets (webpack store immutable flag in assets info).

Breaking Changes

No

Additional Info

Will be great to enable it by default in the next major release.

[codecov]

Codecov Report

Attention: Patch coverage is 36.00000% with 16 lines in your changes missing coverage. Please review.

Project coverage is 58.25%. Comparing base (f7529c3) to head (c4f9e51).
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/middleware.js	23.80%	11 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1924      +/-   ##
==========================================
- Coverage   60.09%   58.25%   -1.84%     
==========================================
  Files          13       13              
  Lines         857      860       +3     
  Branches      244      247       +3     
==========================================
- Hits          515      501      -14     
- Misses        284      296      +12     
- Partials       58       63       +5     

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