Consider factoring of ModelMetadata

[rynowak]

ModelMetadata contains data today used for the following purposes:

• Structure (properties/types)
• Display (data-annotations)
• Validation (data-annotations, requiredattribute)
• Model Binding (bindattribute, frombodyattribute)

To extend the set of provided data or customize discovery, users must author an IModelMetadataProvider, which is very complex.

We should think about the factoring of what data needs to be part of ModelMetadata and what can be moved to a higher level. This could make authoring an IModelMetadataProvider easier, increase cacheability, etc.

[dougbu]

see also #476, #1034, and #1407. there's also a number of low-level bugs about pieces missing from the current ModelMetadata hierarchy e.g. #1000 and #1021.

[rynowak]

90cef3b