Use p2p reference for external ext proj entry point

[jviau]

Issue describing the changes in this PR

resolves #issue_for_this_pr

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

Additional information

• Refactor the method for enabling external ext proj
  • This change was needed to include the ext proj in restore phase. It also is slightly more elegant.
• GenerateExtensionProject target no longer builds, only generates the extension project
• Adds more errors and warnings around correct configuration of external ext proj
  • Error if more than one external ext proj is referenced
  • Error if external ext proj does not exist (but is referenced)
  • Warning if external ext proj does not have ReferenceOutputAssemblies=false.
• Removes Configuration=Release and ManagePackageVersionsCentrally=false from generated csproj
  • Configuration is passed in via global property for non-external scenarios
  • ManagePackageVersionsCentrally was moved to the list of remove-props
• Add sample with docs around external ext proj
  • this will eventually be polished and included in official microsoft docs.
• Update SDK version to 2.1.0-preview.1

[mattchenderson]

Only looking at documentation. Initial pass. Mostly trying to understand the shape of things here.

[brettsam]

ok these docs are awesome...