SqlServer: Support hierarchyid

[bricelam]

This code was created and previously maintained as a community project. Thank you, amazing EF Core community! Now that an official, cross-platform version of Microsoft.SqlServer.Types is available, we intend to support and maintain it as part of the main EF Core project.

Notice, due to our current stance on the in-memory provider, the EntityFrameworkCore.InMemory.HierarchyId package will not be brought forward. If there's enough interest, it may continue as a community project. (Let us know in efcore/EFCore.SqlServer.HierarchyId#120.)

TODO

• Use Microsoft.SqlServer.Types
• Integrate with EF Core codebase
  • Annotate for NRTs
  • Code cleanup
  • Convert internal to pubternal
  • Move tests to existing assemblies (can wait for MQ)
• Review nupkg descriptions

Resolves efcore/EFCore.SqlServer.HierarchyId#113, resolves efcore/EFCore.SqlServer.HierarchyId#119, resolves #365

[bricelam]

Dang it! I just realized the merge commit (7caf9c4) somehow got messed up (6bf7de7) when I rebased this PR to merge it, so all my intentions of preserving the hierarchyid commit history didn't pan out. But at least it was all there for the CLA check.