Search SDK: add reflection-based index definition

[idg10]

This is a feature request. Currently, the Azure Search SDK effectively requires you to define your schema twice, and to ensure that those definitions are in sync. When creating an index, you need to construct an Index object and populate it with a collection of Field objects describing the name and type of each field. Additionally you supply a type representing the index entries when adding or retrieving data. This type must have a property corresponding to each field.

It would reduce duplication and avoid the possibility of inconsistency if the type used when adding and retrieving entries could also be used when creating the index, with the SDK automatically populating the index definition by reflecting against the type. Attributes could be used to indicate which fields should be searchable and which should be the key. (It might make sense to use the .NET data annotation attributes for the latter.)

[brjohnstmsft]

This feature is now merged. Big thanks to @idg10 for contributing this!