Remove unused APIs from BugsnagBreadcrumb interface

[fractalwrench]

Goal

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

Additionally, the BugsnagBreadcrumbs interface has been moved into a separate non-public source file.

Changeset

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

+ (instancetype _Nullable)breadcrumbWithBlock:
    (BSGBreadcrumbConfiguration _Nonnull)block;
+ (instancetype _Nullable)breadcrumbFromDict:(NSDictionary *_Nonnull)dict;

The BugsnagBreadcrumbs class which contains the state of all the breadcrumbs in the application has been moved to a separate non-public source file.

Tests

Relied on existing test coverage