[PLAT-5503] Fix app version reported for crashes before v6.2.3

[nickdowell]

Goal

Fixes an issue whereby crash reports written by earlier releases of Bugsnag (prior to v6.2.3) would be sent to the dashboard with invalid app version information in the payload.

Design

The root cause was that -[BugsnagEvent initWithKSCrashData:] was passing the current configuration to -[BugsnagAppWithState appWithDictionary:config:codeBundleId:], and the app version information therein was being used in preference to the information written by KSCrash (e.g. system[@"CFBundleVersion"]) when user.config did not contain any values (which was the case until v6.2.3 / #862.)

Changeset

-[BugsnagEvent initWithKSCrashData:] now creates a BugsnagConfiguration object initialized with the metadata config JSON written by BugsnagConfiguration in the crashing session.

-[BugsnagApp populateFields:] has been simplified - now that config corresponds to the config for the crashing vesion we no longer need to examine the values in user.config.

Testing

Manually verified by creating a crash report with Bugsnag 6.2.1 and then switching to this branch before building and running the app to send a report.

Some unit tests needed adjusting.

There are E2E test that validate that payloads contain the app versions specified in BugsnagConfiguration:

• features/app_and_device_attributes.feature