Extend ModelMetadata to reduce ugliness

[dougbu]

A couple of potential ModelMetadata additions came up in offline conversations about #5676:

• have ModelMetadata implement IModelMetadataProvider. This should significantly reduce the shenanigans we have around ensuring both ModelMetadata and its provider are available. E.g.
  • many contexts contain both ModelMetadata and IModelMetadataProvider properties.
  • ModelBinderFactory gets the provider from DI though ModelBinderFactoryContext contains a ModelMetadata property.
  • ExpressionMetadataProvider methods contain both ViewDataDictionary (which has ModelMetadata) and IModelMetadataProvider parameters.
• add a ContainerMetadata property. This seem slightly redundant if ModelMetadata implements IModelMetadataProvider but would be convenient.
  • Note information about the containing property is not static and cannot be maintained in ModelMetadata.