* Add Microsoft.AspNetCore.OpenApi package

Fix up schema type generation

Post review polish

Add new package to templates

Split out SchemaGenerator

Clean up usings

* Address feedback from peer review

* Move OpenApi package to top-level directory in src

* Clean up HttpAbstractions filter

* Update ProjectReferences and templates

* Fix up ProjectReferences and address feedback

* Add new directory to Build.props and clean up sln