Add EXIV2_ENABLE_FILESYSTEM_ACCESS option

[enen92]

Hey,

I am trying to add exiv2 as a dependency for Kodi/XBMC as it seems the most complete GPL2 library for image metadata extraction (since it covers both EXIF and IPTC which we currently do with custom parsers - pretty much untouched since the original xbox days ~ 14 years ago). As you might easily guess, it's pretty much broken by now :)

Unfortunately, since the application is available on multiple platforms some of which are not supported by default by exiv2 (e.g. android, tvos, ios, xbox/uwp, etc) filesystem access is not something that can be taken for granted. The usage we plan to do of the library only requires that we are able to read the image (for tag extraction) from a memory buffer - hence we only really need the MemIO.

https://github.com/xbmc/xbmc/pull/24109/files#diff-40b93a8f1858865324006f152285b7b28de2a1b8e878cb9e04ac4f4f4d11dd0fR35-R36

PoC for inclusion is being done in xbmc/xbmc#24109

This PR proposes an additional option EXIV2_ENABLE_FILESYSTEM_ACCESS to allow disable FileIO and generic filesystem access. This is only valid if unit tests, the exiv app, fuzzer, etc are not enable so I think it shouldn't affect the normal behaviour of the application.
In case this is accepted probably the CI should have a way to validate further builds.

[ghost]

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[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (9f90144) 63.88% compared to head (96f9e9a) 63.88%.

Files	Patch %	Lines
src/basicio.cpp	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2837   +/-   ##
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  Coverage   63.88%   63.88%           
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  Files         103      103           
  Lines       22369    22369           
  Branches    10865    10865           
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  Hits        14291    14291           
  Misses       5856     5856           
  Partials     2222     2222           

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[enen92]

Added a new commit to add a new job to validate a build with filesystem disabled. This likely won't run as part of this PR but you can check the result here: https://github.com/enen92/exiv2/actions/runs/6937027229/job/18870288972?pr=2

[enen92]

rebased after #2844 got merged