Update API Approver reporting

[Arkatufus]

The reporting of Akka.API.Tests really needs an overhaul.

1. It relies on a very flaky DiffEngine to try and guess the best diff viewer installed in the system based on the file extension.
2. It assumes that the person running the test can access both the approved.txt and received.txt file to view the diff
3. DiffEngine often fails, resulting in an exception being thrown.
4. There is no diff viewer in the CI pipeline, making debugging impossible.

The better route is to actually embed the diff into the unit test report itself, so that it appears in the failing unit test report.
Diff message format:

<<<<<<<<< [ Approved file name ]
[ Line number ] + This line is newly added
[ Line number ] - This line is removed
[ Line number ] ? This line is modified
=========
[ Line number ] + This line is newly added
[ Line number ] - This line is removed
[ Line number ] ? This line is modified
>>>>>>>>> [ Received file name ]

[Arkatufus]

Example failed output:

Akka.API.Tests.ApiNotApprovedException
Failed API approval. Diff:
<<<<<<<<< CoreAPISpec.ApproveCore.approved.txt
=========
[5060] +     public class static ArrayExtensions
[5061] +     {
[5062] +         public static bool IsNullOrEmpty(this System.Array obj) { }
[5063] +         public static bool NonEmpty(this System.Array obj) { }
[5064] +         public static void Shuffle<T>(this T[] array) { }
[5065] +         public static System.Collections.Generic.Dictionary<T, int> ZipWithIndex<T>(this System.Collections.Generic.IEnumerable<T> collection) { }
[5066] +     }
>>>>>>>>> CoreAPISpec.ApproveCore.received.txt
  Exception doesn't have a stacktrace

[IgorFedchenko]

Does this change anything that is documented here? https://getakka.net/community/public-api-changes.html

If it does not (documentation is up to date, just adds additional output ) - you can treat this PR as approved :)

[Arkatufus]

Nothing is changed, it just adds an extra error reporting when the test catches a public API change, instead of a null referece exception

[IgorFedchenko]

LGTM