[PLAT-4646] Implement discardClasses configuration option

[nickdowell]

Goal

Implements the discardClasses configuration option which contains strings or regular expressions that, when the BugsnagError.errorClass matches, will cause the error to be discarded.

Design

For unhandled errors the evaluation is performed at next launch because it's not possible to evaluate regular expressions in an async-safe manner at crash time.

Changeset

• Added discardClasses property to BugsnagConfiguration
• Added -shouldDiscardErrorClass: method to BugsnagConfiguration
• For handled errors: modified BugsnagClient to discard if -shouldDiscardErrorClass: returns YES
• For unhandled errors: modified BugsnagErrorReportSink to discard if -shouldDiscardErrorClass: returns YES

Testing

• Added unit tests to test -shouldDiscardErrorClass: with a variety of discardClasses values against a set of real-world error classes.
• Added E2E test scenarios to verify that the expected errors are discarded, and that others are not.