ref: move decl/impl from SentryEvent (MetricKit) to SenryEvent .h/.m …

[armcknight]

Another conversion of a category to interface extension (see #3236 for elaboration)

This also moved the extension interface and implementation to SentryEvent instead of being hidden in SentryMetricKitIntegration. It also declared isMetricKitEvent as a method since that's what it actually is, and converted callsites to use method syntax.

#skip-changelog

[codecov]

Codecov Report

Merging #3240 (76c2c08) into main (01a28a9) will increase coverage by 0.047%.
The diff coverage is 100.000%.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3240       +/-   ##
=============================================
+ Coverage   89.112%   89.159%   +0.047%     
=============================================
  Files          502       502               
  Lines        54118     54104       -14     
  Branches     19429     19419       -10     
=============================================
+ Hits         48226     48239       +13     
+ Misses        5029      5007       -22     
+ Partials       863       858        -5     

Files Changed	Coverage Δ
Sources/Sentry/SentryMetricKitIntegration.m	99.578% <ø> (-0.027%)	⬇️
Sources/Sentry/SentryEvent.m	98.473% <100.000%> (+0.212%)	⬆️
Sources/Sentry/SentryScreenshotIntegration.m	90.697% <100.000%> (ø)
Sources/Sentry/SentryViewHierarchyIntegration.m	100.000% <100.000%> (ø)
...grations/MetricKit/SentryMetricKitEventTests.swift	100.000% <100.000%> (ø)

... and 10 files with indirect coverage changes

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[brustolin]

LGTM