Serialize preContext and postContext in SentryStackFrame

[lcian]

📜 Description

Serializes and deserializes the preContext and postContext fields in SentryStackFrame.
They don't seem to be used anywhere in the SDK, so perhaps that's why they were ignored before.

💡 Motivation and Context

Closes #4479

💚 How did you test it?

Updated existing test.

📝 Checklist

• 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.

🔮 Next steps

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	401.53 ms	432.71 ms	31.18 ms
Size	1.58 MiB	2.12 MiB	548.76 KiB

Previous results on branch: lcian/feat/serialize-frame-context

Startup times

Revision	Plain	With Sentry	Diff
843440a	444.15 ms	513.04 ms	68.89 ms

App size

Revision	Plain	With Sentry	Diff
843440a	1.58 MiB	2.08 MiB	511.19 KiB

[romtsn]

lgtm, but can we please add a changelog entry? Got bitten a few times where we didn't mention things and I was looking for them

[lcian]

lgtm, but can we please add a changelog entry? Got bitten a few times where we didn't mention things and I was looking for them

Thanks, sure, adding it!