Allow addition/removal of OnBreadcrumb callbacks

[fractalwrench]

Goal

Provides a method for adding/removing callbacks which can mutate every Breadcrumb before it is added to the application. This matches the notifier specification.

Changeset

• Added addOnBreadcrumbBlock and removeOnBreadcrumbBlock to Bugsnag and BugsnagConfiguration
• Stored OnBreadcrumb callbacks in BugsnagBreadcrumbs
• Executed callbacks before adding a breadcrumb and discarded the crumb if false is returned from any callback

Tests

Added new test coverage for adding/removing callbacks, and verifying that the callback discards/mutates breadcrumbs.