feat: pointer-ize model optional properties

Make model optional value be pointer to enable distinguish
omitting JSON fields and Go zero values.

This approach cannot distinguish NotSet and Null.
See: <OpenAPITools/openapi-generator#522>

BREAKING CHANGE: model optional properties are pointer-ize,
so partial model property types will be changed.

src/main/resources/go-middleware-server/openapi/model.mustache

@@ -19,6 +19,6 @@ const (
 type {{classname}} struct {
 {{#vars}}{{#description}}
  // {{{description}}}{{/description}}
- {{name}} {{#isNullable}}*{{/isNullable}}{{{dataType}}} `json:"{{baseName}}{{^required}},omitempty{{/required}}"{{#vendorExtensions.x-go-custom-tag}} {{{.}}}{{/vendorExtensions.x-go-custom-tag}}`
+ {{name}} {{#isNullable}}*{{/isNullable}}{{^isNullable}}{{^required}}*{{/required}}{{/isNullable}}{{{dataType}}} `json:"{{baseName}}{{^required}},omitempty{{/required}}"{{#vendorExtensions.x-go-custom-tag}} {{{.}}}{{/vendorExtensions.x-go-custom-tag}}`
 {{/vars}}
 }{{/isEnum}}{{/model}}{{/models}}