Add support for specifying non-clustered index sort direction

[gumbarros]

This closes #636

[ckadluba]

Thank you for the PR @gumbarros! After a first glance at the code, it looks quite good. A few tings are still to be done.

1. There is one of more failing tests.
2. The new config parameter also has to be read from config files (check out the code in src/Serilog.Sinks.MSSqlServer/Configuration)
3. Add tests for the new functionality (especially in SqlTableWriter)

[gumbarros]

Yw @ckadluba, sorry for the late response.

I have added support for reading the new config parameter from the config files. However, I'm not very familiar with the test setup and I'm feeling a bit overwhelmed by the complexity of fixing and writing the tests. I'd really appreciate some help from someone with that part.