[Feature request] Create default implementation for JSchemaGenerationProvider

[Drake53]

Currently, the GetSchema method in JSchemaGenerationProvider is abstract. This makes it harder to implement a custom provider, because you can't simply call base.GetSchema(context); and add something on top of that.

As a workaround, I now implemented the custom provider as follows:

public override JSchema GetSchema(JSchemaTypeGenerationContext context)
{
  context.Generator.GenerationProviders.Remove(this);
  var schema = context.Generator.Generate(context.ObjectType, context.Generator.DefaultRequired, context.MemberProperty);
  context.Generator.GenerationProviders.Add(this);

  // custom generator implementation

  return schema;
}

It would be nice if this could be simplified to:

public override JSchema GetSchema(JSchemaTypeGenerationContext context)
{
  var schema = base.GetSchema(context);

  // custom generator implementation

  return schema;
}

Note that the workaround does not work when creating a schema for an object with nested entities, because then the custom generation provider is only applied to the root object.