ref: Add SentryMechanismMeta

[philipphofmann]

📜 Description

Replace the dictionary for the mechanism meta for the exception with a
class. Furthermore, move the SentryNSError to SentryMechanismMeta.

💡 Motivation and Context

We want to align the SDK with the exception interface.

💚 How did you test it?

Unit tests and simulator

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the CHANGELOG if needed
• I updated the docs if needed
• Review from the native team if needed
• No breaking changes

🔮 Next steps

[codecov-io]

Codecov Report

Merging #1038 (7c0f660) into master (f0eb381) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1038      +/-   ##
==========================================
+ Coverage   94.86%   94.88%   +0.01%     
==========================================
  Files          92       93       +1     
  Lines        4074     4083       +9     
==========================================
+ Hits         3865     3874       +9     
  Misses        209      209              

Impacted Files	Coverage Δ
Sources/Sentry/SentryClient.m	94.46% <100.00%> (+0.04%)	⬆️
Sources/Sentry/SentryCrashReportConverter.m	95.03% <100.00%> (ø)
Sources/Sentry/SentryMechanism.m	100.00% <100.00%> (ø)
Sources/Sentry/SentryMechanismMeta.m	100.00% <100.00%> (ø)
Sources/Sentry/SentryCrashIntegration.m	100.00% <0.00%> (ø)

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[bruno-garcia]

LGTM