Migrate Country.flag() from deprecated flags.fmcdn.net to flagcdn.com; add overload

[spannm]

Motivation

flags.fmcdn.net is deprecated. The domain operator explicitly requests migrating to flagcdn.com:

Flags.fmcdn.net was CDN of country flags for Flagpedia.net. If you use this domain for embedding flags to your own website, please make the change from flags.fmcdn.net to flagcdn.com that we created exactly for this purpose.

Changes

Country.java

• Replace deprecated flags.fmcdn.net base URL with https://flagcdn.com/
• Replace instance field flagUrl by static final FLAG_BASE_URL
• Add ImageFormat enum (JPG, PNG, SVG, WEBP)
• Add overload flag(String size, ImageFormat format) for full control over image size and format

CountryTest.java

• Update testFlag() regex to match the new URL pattern
• Add testFlagWithSizeProducesValidUrl (@ValueSource) to verify URL structure for a number of sample sizes
• Add testFlagWithFormatProducesValidUrl (@EnumSource) - covers all ImageFormat values

Backward Compatibility

flag() without arguments keeps the same behavior as before (PNG, width 580px).
Only the CDN domain changes, so existing usages are unaffected.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.52%. Comparing base (ce0f7cf) to head (fb522f1).
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Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1857      +/-   ##
============================================
+ Coverage     92.41%   92.52%   +0.11%     
- Complexity     3555     3561       +6     
============================================
  Files           345      345              
  Lines          7025     7037      +12     
  Branches        685      686       +1     
============================================
+ Hits           6492     6511      +19     
+ Misses          367      364       -3     
+ Partials        166      162       -4     

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[bodiam]

Thanks @spannm !