Add some typescript definitions

[efbenson]

When doing things like creating custom providers it would be nice to have typescript definitions for all the data that is returned through the getData function. In particular the configuration data like meta, relationships etc. With that it would be easier for the developers to know what is required or can be used for different scenarios. Taking away some of the problems away when you can spend only some stuff back but if you don't send enough information back about the dataset you get odd errors that are not easy to debug. Mostly I think the types for the configuration information in Featureserver being the most prominent. I have some examples that I have thrown in my local project to help us along.

  export interface Relationship {
    id: number, // The unique relationship id.
    name: string, // The name of the relationship
    relatedTableId: number, // Id of the layer/table related records are found
    cardinality: string, // esriRelCardinalityOneToMany | esriRelCardinalityManyToMany
    role: string, // esriRelRoleOrigin | esriRelRoleDestination
    keyField: string, // key field name in the related Table
    composite: boolean// likely to false
  }

  export interface Metadata {
    id: number,
    name: string
    description: string
    extent?: number[][]
    displayField?: string
    geometryType?: string
    idField?: string
    maxRecordCount: number
    limitExceeded?: boolean
    fields: Field[]
  }