Support adding/removing table headers and footers

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)

Start by reading the CdkTable implementation, especially its content query for header and footer definitions. Compare the proposed approaches: watching definition changes and re-rendering, or supporting dynamic changes through when or a similar API. Done means users can add or remove table headers and footers dynamically so the corresponding elements are no longer left in the DOM.

Written by the indexing model from the issue text.