Support adding/removing table headers and footers

[jelbourn]

Currently CdkTable doesn't offer a way to add/remove headers and footers dynamically. Users can hide and show them with CSS, but they always remain in the DOM.

I think we could support this in one of two ways:

1. Add some code to the content query for defs to watch for changes and re-render
2. Support doing this with when (or maybe a similar API)