Support optional parameters in blazor routing

[ichensky]

Routing with nulable types not working.

1. Create blazor page.
2. Add route 'fetchdata/{page:int?}'
   There will be an exception:

Invalid template 'fetchdata/{page:int?}'. 
The character '?' in parameter segment '{page:int?}' is not allowed.

Also it is not possible create "double routing" like in the issue #10686

[SteveSandersonMS]

Note: when we do this, we need to make sure it supplies null for any omitted optional parameters for consistency with #12407.