chore: Remove SentryNetworkTracker singleton

[itaybre]

📜 Description

Follow up to remove SentryNetworkTracking singleton.
After testing this more, I noticed that at least some kind of singleton is needed because of the swizzling + closing and enabling again the SDK.

I found another way to implement this.
But I am not sure this is the way to go, because in the end I traded one singleton for another.

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

You have to check all boxes before merging:

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.746%. Comparing base (406b0d4) to head (a39c44a).
⚠️ Report is 34 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #7107       +/-   ##
=============================================
+ Coverage   84.714%   84.746%   +0.032%     
=============================================
  Files          459       459               
  Lines        27496     27508       +12     
  Branches     12121     12125        +4     
=============================================
+ Hits         23293     23312       +19     
+ Misses        4160      4155        -5     
+ Partials        43        41        -2     

Files with missing lines	Coverage Δ
SentryTestUtils/Sources/ClearTestState.swift	82.926% <ø> (-0.407%)	⬇️
Sources/Sentry/SentryNetworkTracker.m	100.000% <ø> (ø)
Sources/Sentry/SentrySDKInternal.m	84.583% <100.000%> (-0.064%)	⬇️
Sources/Sentry/SentrySwizzleWrapperHelper.m	100.000% <100.000%> (ø)
Sources/Swift/Core/Integrations/Integrations.swift	100.000% <100.000%> (ø)
...nce/Network/SentryNetworkTrackingIntegration.swift	100.000% <100.000%> (ø)

... and 5 files with indirect coverage changes

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1222.81 ms	1247.62 ms	24.82 ms
Size	24.14 KiB	1.03 MiB	1.00 MiB

Baseline results on branch: itay/cocoa-993-refactor-sentrynetworktrackingintegrationm-in-swift

Startup times

Revision	Plain	With Sentry	Diff
3ad0bf4	1224.53 ms	1243.49 ms	18.96 ms

App size

Revision	Plain	With Sentry	Diff
3ad0bf4	24.14 KiB	1.03 MiB	1.00 MiB

Previous results on branch: itay/remove_networktracker_singleton

Startup times

Revision	Plain	With Sentry	Diff
23ba5b4	1221.09 ms	1246.98 ms	25.89 ms

App size

Revision	Plain	With Sentry	Diff
23ba5b4	24.14 KiB	1.03 MiB	1.00 MiB

[github-actions]

🚨 Detected changes in high risk code 🚨

High-risk code can easily blow up and is hard to test. We had severe bugs in the past. Be extra careful when changing these files, and have an extra careful look at these:

• Sources/Sentry/SentryNetworkTracker.m