Make some enums nullable

[EPRenaud]

In C#, enums are just glorified int and are therefore not nullable. The default value is the associated enum value of the int 0.

Fix https://github.com/FusionAuth/fusionauth-netcore-client/issues/31

[robotdan]

Thanks @EPRenaud - this seems like a good idea. Ideally we'd solve this for all enum's.

For example, the JSON DSL for the domain object will look something like this:

{
  "packageName" : "io.fusionauth.domain",
  "type" : "KeyType",
  "enum" : [ "EC", "RSA", "HMAC" ]
}

I wonder if we can use the enum field value here instead of type to make this nullable so that we cover all enums.

I can accept the PR and then investigate if we can add a more comprehensive change to make enums nullable.