Implement deep copy for OpenApiSchema instances

[captainsafia]

Closes #56990.

As the last transformation we apply to the OpenAPI document, we utilize the OpenApiSchemaReferenceTransformer to replace inlined schemas in the documents with reference schemas that reference the schema definition in the top-level document.schemas registery. As part of this process, we map schemas store in the OpenApiSchemaStore cache to elements in the OpenAPI document's components.schemas property as seen below.

aspnetcore/src/OpenApi/src/Transformers/Implementations/OpenApiSchemaReferenceTransformer.cs

Lines 18 to 37 in 257d690

	var schemaStore = context.ApplicationServices.GetRequiredKeyedService<OpenApiSchemaStore>(context.DocumentName);
	var schemasByReference = schemaStore.SchemasByReference;
	document.Components ??= new OpenApiComponents();
	document.Components.Schemas ??= new Dictionary<string, OpenApiSchema>();
	foreach (var (schema, referenceId) in schemasByReference.Where(kvp => kvp.Value is not null).OrderBy(kvp => kvp.Value))
	{
	// Reference IDs are only set for schemas that appear more than once in the OpenAPI
	// document and should be represented as references instead of inlined in the document.
	if (referenceId is not null)
	{
	// Note: we create a copy of the schema here to avoid modifying the original schema
	// so that comparisons between the original schema and the resolved schema during
	// the transformation process are consistent.
	document.Components.Schemas.Add(
	referenceId,
	ResolveReferenceForSchema(new OpenApiSchema(schema), schemasByReference, isTopLevel: true));
	}
	}

The implementation currently uses the copy constructor implementation on OpenApiSchema to avoid making mutations to instances in the cache that we copy from that are used later in the application.

aspnetcore/src/OpenApi/src/Transformers/Implementations/OpenApiSchemaReferenceTransformer.cs

Lines 30 to 36 in 257d690

	// Note: we create a copy of the schema here to avoid modifying the original schema
	// so that comparisons between the original schema and the resolved schema during
	// the transformation process are consistent.
	document.Components.Schemas.Add(
	referenceId,
	ResolveReferenceForSchema(new OpenApiSchema(schema), schemasByReference, isTopLevel: true));
	}

However, the copy constructor implementation for OpenApiSchema executes a shallow copy on sub-schemas within a top-level OpenApiSchema which means that modiciations to the copy made in ResolveReferenceForSchema end up impacting the instances in the cache.

To resolve this issue, we implement a deep copy implementation for OpenApiSchema so that subschemas in the copied instance can be modified without impacting the original instance.