Add OSS-Fuzz status badge and CIFuzz integration

[kevinbackhouse]

• Add the OSS-Fuzz badge to the README.
• Update the documentation to explain the CMake option EXIV2_TEAM_OSS_FUZZ.
• Add the CIFuzz workflow.

[clanmills]

Excellent. Great idea to add the Fuzzing Status Icon.

[codecov]

Codecov Report

Merging #1876 (3587618) into main (079f130) will not change coverage.
The diff coverage is n/a.

❗ Current head 3587618 differs from pull request most recent head 5bfa568. Consider uploading reports for the commit 5bfa568 to get more accurate results

@@           Coverage Diff           @@
##             main    #1876   +/-   ##
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  Coverage   60.77%   60.77%           
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  Files          96       96           
  Lines       18887    18887           
  Branches     9498     9498           
=======================================
  Hits        11478    11478           
  Misses       5116     5116           
  Partials     2293     2293           

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