feat(nullable-validation): Add validation parameter to enable "nullable" values (eg String or null)

[TiFu]

First time contributing to RESTinstance - so in case I missed something, let me know! :)

This PR introduces a new feature to allow checks for values that can be null (e.g. to assert a values is either string or null).

This is implemented as a non-breaking change, i.e. current behavior of not accepting null is maintained.

Example usage:

String response body  nullable=True # Validate whether `response.body == null || response.body is "string"
Integer response body  nullable=True # Validate whether `response.body == null || response.body is "integer"
Array response body  nullable=True # Validate whether `response.body == null || response.body is "array"
Object response body  nullable=True # Validate whether `response.body == null || response.body is "object"
Number response body  nullable=True # Validate whether `response.body == null || response.body is "number"

• Test cases for _set_type_validations have been added both base case and nullable
• Documentation is updated to reflect this new option

[asyrjasalo]

Great!