System.Data.Odbc future improvements

[colltoaction]

Now that dotnet/corefx#15646 is (about to be) merged, let's track future work here:

1. Enable tests: please see Add Odbc and SQLite OS deps to CI #20097 for info on the missing dependencies
2. Possible C types mismatch:
   • Per http://www.unixodbc.org/doc/ProgrammerManual/Tutorial/gloss.html the native types are not fixed-width
   • Currently everything works fine in our supported platforms, but we might end up needing a shim such as here https://github.com/dotnet/corefx/tree/master/src/Native/Unix/System.Native
3. Publish as a Nuget package in the nightly feed
4. Document steps to install native dependencies

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