Remove unused APIs on BugsnagEvent interface

[fractalwrench]

Goal

Removes/hides unused APIs from the BugsnagEvent interface. These are either no longer required or were originally part of the internal implementation and mistakenly included in the public header.

Changeset

The following APIs have been removed/hidden using a class extension:

BSGParseSeverity
BSGFormatSeverity
serializableValueWithTopLevelData
shouldBeSent
toJson
attachCustomStacktrace:withType:
enhancedErrorMessageForThread
notifyReleaseStages
handledState
overrides
depth
error
isIncomplete

Tests

Updated existing test coverage to avoid using deprecated APIs. Mazerunner scenarios which were testing individual stackframes have had these assertions removed as historically this could be quite flaky.

[tomlongridge]

It looks like we've had to remove some of the test rigor in this change now that we've made some field private. Can we not use some class extensions to give access in the tests?

[fractalwrench]

After IRL discussion attachCustomStacktrace has been reinstated as it was before, as this allows us to retain the same test coverage. It will be removed as part of future work once the Error interface has been implemented and the existing scenarios can be updated to use this API.