APIs to read auditSources from nuget.config files

[zivkan]

Bug

Fixes: NuGet/Home#13211

Regression? no

Description

• Make ConfigurationDefaults be able to load audit sources from NuGetDefaults.config
• Add LoadAuditSources() to IPackageSourceProvider and PackageSourceProvider`
• #nullable enable in several files
• Fix a bunch of tests that fail when your machine has a NuGetDefaults.config with at least one package source defined. Tests should not depend on machine state.

The only functionality proposed at the moment is to read the config, not to add/modify/delete auditSources, so no APIs have been added for those actions yet.

PR Checklist

• PR has a meaningful title

• PR has a linked issue.

• Described changes

• Tests

  • Automated tests added
  • OR
  • Test exception
  • OR
  • N/A

• Documentation

  • Documentation PR or issue filled: https://github.com/NuGet/docs.microsoft.com-nuget/issues/3226
  • OR
  • N/A