Replace FixtureConfig with an Objective-C version

[kstenerud]

Goal

Using a Swift version of FixtureConfig makes objc interop harder because you have to forward declare the class in objc, which makes splitting out certain parts of the interface impossible.

Instead, we just make an objective-c version. It's just a data class, so bridging header interop works fine.

This also splits out the bridging header to have one that accesses internal APIs, and another that only has access to our public API (for XCFramework based testing fixtures).

[github-actions]

Bugsnag.framework binary size did not change - 656,120 bytes

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