Feaure: Make it possible to not have a default enum value and ignore a property

**Describe the issue**
By default, when converting an enum, the first index will be considered as the default enum property. However, in some cases, the enum properties are optional meaning that there should not be a default value.
More over, while coding I noticed that I was looking for a way to customize what should be included in the OpenApi Spec, but did not succeed in my research - I have tried using the annotations [JsonIgnore] and [IgnoreDataMember], but that did not affect anything. I have read the [documentation](https://github.com/Azure/azure-functions-openapi-extension/blob/main/docs/openapi-core.md#displayattribute) about how to show the string value of the enum properties, but unfortunately, it does always include everything and it is (seemingly) not possible to handpick, which should not be included.

**To Reproduce**

**Expected behavior**
Data annotations that enables one to tell there is no default value and exclude some properties from the spec.

**Environment (please complete the following information, if applicable):**
- OS: Windows
- .Net version: 6

Start with the openapi-core.md DisplayAttribute documentation linked in the issue, then trace how enum properties are converted into the OpenAPI spec. Define how annotations should express no default enum value and how selected properties should be excluded, and verify that both behaviors are reflected in the generated spec.

Written by the indexing model from the issue text.