PathString should have a registered TypeConverter

[MarkPflug]

I ran into an issue while attempting to configure the OpenIdConnect provider using the Options/configuration APIs. The issue was around trying to assign a the OpenIdConnect.CallbackPath value which is of type PathString. This failed, with a conversion error trying to convert my string value from the configuration to the PathString property.
The original issue is here: aspnet/Security#1230

This can resolved by reigistering a TypeConverter for PathString. However, it seems like such a TypeConverter would be best defined along side the PathString class itself.

[HaoK]

Added for preview 2, but I need to harden the implementation + add a few more tests, so keeping this on my plate for now

[HaoK]

12f89f6