Allow binding row-island `key` property to complex object.

[MayaKirova]

Is your feature request related to a problem? Please describe.

Currently it is not possible to bind the row island to a complex object via just the key property. You could do a manual bind via events but that seems inconvenient.

Describe the solution you'd like

Ability to set the the key of the row-island to a complex object. For example:

<igx-row-island key='Records.Developers' ... >

Similar to the complex object binding of field in the columns: https://www.infragistics.com/products/ignite-ui-angular/angular/components/grid/grid#complex-data-binding