Fix Web API schema parsing problems

[Michael-Herzog]

• Circular references. There seems to be a massive problem with navigation properties.
  Validators like this report circular references.
• The Operation ID of endpoints seems to be wrong. I couldn't find the place in the specification where it says there are forbidden characters, but all samples I saw do it in lower camel case (e.g. mySuperDuperOperationId). Some validators have problems with special chars like / which is contained in every single one of our operation ids.
• The latest version of Open API is 3.1.0 we're using 3.0.1
• "maxLength": -1, which describes fields with nvarchar(max) in our json is not valid.

[mgesing]

Circular references. There seems to be a massive problem with navigation properties.
Validators like this report circular references.

The validation message is "This API is valid, but it cannot be shown because it contains circular references", for example in case of Country > StateProvinces. This is a bug in the validator when converting these structures to JSON. Circular model references are quite common and certainly there is nothing wrong with them. The question is, how do a validator deal with it? Swashbuckle de-reference it via an option DefaultModelExpandDepth, which is set to 2 in our case.