Copy constructor doesn't create new `OpenApiPathItem` objects when creating new instance of `OpenApiPaths`

[irvinesunday]

Describe the bug
Copy constructors doesn't create a new instance of OpenApiPathItem and its constituent properties on new OpenApiDocument(document).

To Reproduce
I have created a breaking test to demonstrate this:

OpenAPI.NET/test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs

Lines 1341 to 1358 in d5a5f8a

	[Fact]
	public void CopyConstructorForAdvancedDocumentWorks()
	{
	// Arrange & Act
	var doc = new OpenApiDocument(AdvancedDocument);
	// Change value of operation id for a given url
	var docOpId = doc.Paths["/pets"].Operations[OperationType.Get].OperationId = "findAllMyPets";
	var advancedDocOpId = AdvancedDocument.Paths["/pets"].Operations[OperationType.Get].OperationId;
	// Assert
	Assert.NotNull(doc.Info);
	Assert.NotNull(doc.Servers);
	Assert.NotNull(doc.Paths);
	Assert.Equal(2, doc.Paths.Count);
	Assert.NotNull(doc.Components);
	Assert.NotEqual(docOpId, advancedDocOpId);
	}

This is in the branch: is/copy-ctrs

Expected behavior
In the test, I change the operationId of the cloned OpenAPI document. I don't expect the operationId of the original OpenAPI document to also be updated.

[darrelmiller]

When cloning OpenApiPaths we are using the standard Dictionary constructor which does not do a clone of the values of the key value pairs.
https://github.com/microsoft/OpenAPI.NET/blob/vnext/src/Microsoft.OpenApi/Models/OpenApiExtensibleDictionary.cs#L31

[MaggieKimani1]

Reopening this issue as its not fixed.