updating for v4

[brettsam]

Lots of changes here to support v4 builds and versioning:

• Everything netcoreapp3.1 -> net6.0
• Changed build to be fully azure pipelines (no more F# build script or packet)
• Updated versioning to support:
  • previews
  • CI build numbers appending
  • SourceLink
  • Generator and MSBuild now have same versions as nuget package
  • No longer have hardcoded props file with version (we now set FunctionsSdkVersion during build init)
• Removed Generator -> Sdk dependency. Generator now gets version from itself (since it is built with same versions)
• E2E tests now explicitly add the newly-packed package rather than using hardcoded version.
• Removed v1 tests.

[brettsam]

Since this is a preview and is such a re-do of the build system, I'm going to get this in and please let me know @soninaren if you have additional feedback; we can address directly in the branch in separate PRs.