Integrate diffing into build

Generate diff artifact as part of es.net build script.

• Alert if Type="Deleted" or Type="Modified" is present
• Attempt to de-duplicate erroneous Type="Modified" on types
• Potentially filter on Modified and Deleted to aid human-check

[Mpdreamz]

We use: https://github.com/nullean/AssemblyDiffer which handles TFM’s as well.

It allows us to compare the current build assemblies with a version on nuget.

The trouble is we need to feed that tool previous Nuget versions manually since depending on what branch we are we need a different version to compare against

branch	version	action
5.x	latest 5.x on nuget.org	fail if breaking
6.x	latest 6.x on nuget.org	fail if breaking
7.x	latest 7.x on nuget.org	fail if breaking
master	latest on nuget.org	break all the things!

The version resolution requires investigating how Nuget.Client and its N auxialiary packages stitch together, things were a tad easier when it was OData. We currently use the v2 OData endpoints and but they’ve been deprecated for years now.

[russcam]

Closing this as this change has now been made in 6.x and 7.x. For example

> build skipdocs skiptests
> build diff "nuget|NEST.JsonNetSerializer|6.6.0|netstandard2.0 directory|../../build/output/Nest.JsonNetSerializer/netstandard2.0"

This will

1. pull down the Nest.JsonNetSerializer 6.6.0 nuget package and its dependent packages
2. compare all matching assembly names in the package and dependent packages with those in the …/…/build/output/Nest.JsonNetSerializer/netstandard2.0 build output directory. In this instance, it will compare Elasticsearch.Net, Nest and Nest.JsonNetSerializer
3. output the diff XML for each to build/output

We can aim to make this even easier to run in future so that you can just call something like

build diff "nuget|6.x|netstandard2.0"

to compare local compiled with the latest 6.x released version.