[ci] Add support for generating `nuget-diff.md`.

[jpobst]

Context: https://github.com/mattleibow/Mono.ApiTools.NuGetDiff/pull/17

Creates a new CI artifact: api-diff/all-nuget-diff.md.

This file lists all changes made to the structure of NuGet packages built by this repository. This will help give us extra confidence that we are not accidentally making breaking changes like removing a needed .targets file.

This includes:

• Changes to which files are included in the package. (Note this is only presence of files, it does not compare file contents.)
• Changes to NuGet package metadata.

For example, adding support for net7.0-android to the Xamarin.AndroidX.Core package looks like this:

## Xamarin.AndroidX.Core

### Added/Removed File(s)

+ lib/net7.0-android33.0/Xamarin.AndroidX.Core.pdb
+ lib/net7.0-android33.0/Xamarin.AndroidX.Core.dll
+ lib/net7.0-android33.0/Xamarin.AndroidX.Core.xml
+ build/net7.0-android33.0/Xamarin.AndroidX.Core.targets
+ buildTransitive/net7.0-android33.0/Xamarin.AndroidX.Core.targets

Note: we are currently waiting on a new official published package of api-tools that contains this new support. In the meantime this uses a locally built package called private-api-tools.