fix: Synchronize C# metadata changes with native layer

[kattrali]

Goal

When using Bugsnag.Metadata.Add(), any added metadata should be synchronized with the native layer to ensure the values are sent with native crash reports as well as Unity log reports.

Example usage:

void Start() {
    Bugsnag.Metadata.Add("app", new Dictionary<string, string>(){
      {"buildno", "0.1" },
      {"package", "C" }
    });
}

Changeset

• Added a INativeClient ref to the Metadata initializer
• Enhanced the Metadata class with custom Add() and Remove() implementations which propagate changes to the native layer
• Switches the Report.Metadata class to Dictionary<string, object> instead of Metadata since it requires no synchronization

Requires bugsnag/bugsnag-android#505

Artifacts

• Bugsnag.unitypackage.zip (md5: 7ff0149c7d62ab8bf74bac0650aa6190, sha1: 2e234891e3c8217ee677e401b75a481afa9f0aae)
• Bugsnag-with-android-64bit.unitypackage.zip (md5: b900643ada1ca9d7d932fd742be45cd0, sha1: 408e9ac22c375d6efc41f664ad1716fdd25a2e26)

Tests

Test handled and unhandled, native and C# crashes on each:

• Test change on iOS
• Test change on macOS
• Test change on Android

[fractalwrench]

Left a few comments that will need resolution before releasing. It'd be good to understand why Report.MetaData was changed to a dictionary - as this appears to be a breaking change?

[kattrali]

Made a few changes to make the fix less invasive. Metadata now has a non-native-syncing variant.