WIP: Add code analyzers

[nbarbettini]

Adding the code analyzers used in Okta.Sdk to keep a consistent style.

• Add StyleCop.Analyzers and configuration
• Add AsyncUsageAnalyzers
• Add analyzers to non-.NET Standard projects
• Fix warnings

[nbarbettini]

Background: When writing Okta.Sdk, I used StyleCop.Analyzers to enforce good code style and keep all of my code consistent. I'd like to do the same thing here, and use the same style rules from Okta.Sdk. In this PR, I copied the style rules from the SDK project and enabled the analyzers in each project.

Tl;dr: Now there are tons of warnings and errors to fix in all the projects. (This is normal when adding analyzers!) The remaining work is to get rid of all the warnings. 😄 Most of them have automatic code fixes (lightbulb icon) in Visual Studio!