Fix image flickering when using SentryAssetBundle

[denrase]

📜 Description

We cannot await within futures, or asset loading will lead to "flickering". This PR replaces async/await with Completers who check if they are sync from the call order.

💡 Motivation and Context

Closes #2528

💚 How did you test it?

Unit tests

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov]

Codecov Report

Attention: Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.49%. Comparing base (a22e451) to head (e003cac).

Files with missing lines	Patch %	Lines
flutter/lib/src/sentry_asset_bundle.dart	95.58%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2577      +/-   ##
==========================================
+ Coverage   88.96%   92.49%   +3.52%     
==========================================
  Files         262       91     -171     
  Lines        8957     2970    -5987     
==========================================
- Hits         7969     2747    -5222     
+ Misses        988      223     -765     

☔ View full report in Codecov by Sentry.
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[denrase]

@buenaflor Do we have some larger sample that is loading multiple assets where we known hot the spans are supposed to look like on sentry.io?

[github-actions]

Android Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	452.52 ms	492.86 ms	40.34 ms
Size	6.46 MiB	7.48 MiB	1.02 MiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
dd76eef	461.37 ms	540.55 ms	79.18 ms
e8603bb	377.51 ms	444.77 ms	67.26 ms
0a82a1e	321.02 ms	393.82 ms	72.80 ms
72eeb80	421.38 ms	520.81 ms	99.44 ms
2f8f173	323.31 ms	373.29 ms	49.97 ms
d10745a	318.86 ms	374.82 ms	55.96 ms
84bc635	395.57 ms	464.23 ms	68.66 ms
8932ece	309.56 ms	377.28 ms	67.72 ms
633cf2e	289.36 ms	340.38 ms	51.02 ms
4b943a1	348.17 ms	437.15 ms	88.98 ms

App size

Revision	Plain	With Sentry	Diff
dd76eef	6.35 MiB	7.40 MiB	1.05 MiB
e8603bb	6.34 MiB	7.28 MiB	967.80 KiB
0a82a1e	6.15 MiB	7.11 MiB	981.82 KiB
72eeb80	6.35 MiB	7.42 MiB	1.06 MiB
2f8f173	5.94 MiB	6.95 MiB	1.01 MiB
d10745a	6.26 MiB	7.20 MiB	956.08 KiB
84bc635	6.34 MiB	7.28 MiB	968.41 KiB
8932ece	6.16 MiB	7.13 MiB	1000.89 KiB
633cf2e	5.94 MiB	6.92 MiB	1001.53 KiB
4b943a1	6.34 MiB	7.28 MiB	968.41 KiB

Previous results on branch: fix/sentry-asset-bundle-flickering

Startup times

Revision	Plain	With Sentry	Diff
ba7f41b	449.35 ms	522.44 ms	73.09 ms
a24e107	460.25 ms	537.63 ms	77.38 ms

App size

Revision	Plain	With Sentry	Diff
ba7f41b	6.46 MiB	7.48 MiB	1.03 MiB
a24e107	6.46 MiB	7.48 MiB	1.03 MiB

[github-actions]

iOS Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1253.83 ms	1262.25 ms	8.42 ms
Size	8.42 MiB	9.89 MiB	1.47 MiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
90db9ff	1277.18 ms	1283.69 ms	6.51 ms
1a93825	1257.25 ms	1261.55 ms	4.30 ms
905bf99	1240.84 ms	1271.47 ms	30.63 ms
2d74010	1264.45 ms	1268.42 ms	3.97 ms
e5b744f	1250.82 ms	1284.46 ms	33.64 ms
3e9fb0e	1262.49 ms	1280.65 ms	18.16 ms
7045efc	1246.45 ms	1262.24 ms	15.80 ms
09eab47	1246.88 ms	1267.71 ms	20.83 ms
8609bd8	1267.16 ms	1291.39 ms	24.22 ms
33d0587	1262.16 ms	1270.50 ms	8.34 ms

App size

Revision	Plain	With Sentry	Diff
90db9ff	8.10 MiB	9.08 MiB	1004.27 KiB
1a93825	8.28 MiB	9.34 MiB	1.05 MiB
905bf99	8.38 MiB	9.74 MiB	1.36 MiB
2d74010	8.32 MiB	9.38 MiB	1.05 MiB
e5b744f	8.09 MiB	9.07 MiB	1001.19 KiB
3e9fb0e	8.15 MiB	9.12 MiB	989.77 KiB
7045efc	8.38 MiB	9.78 MiB	1.40 MiB
09eab47	8.38 MiB	9.77 MiB	1.40 MiB
8609bd8	8.28 MiB	9.34 MiB	1.06 MiB
33d0587	8.29 MiB	9.38 MiB	1.09 MiB

Previous results on branch: fix/sentry-asset-bundle-flickering

Startup times

Revision	Plain	With Sentry	Diff
ba7f41b	1244.04 ms	1259.22 ms	15.18 ms
a24e107	1251.55 ms	1269.35 ms	17.80 ms

App size

Revision	Plain	With Sentry	Diff
ba7f41b	8.42 MiB	9.89 MiB	1.46 MiB
a24e107	8.42 MiB	9.89 MiB	1.46 MiB

[buenaflor]

@denrase hmm I don't think we have a larger sample, I could try to check if we have customer data on that