Create selectors to retrieve admin-menu data from state

[getdave]

This is a Sub Issue of #45435.

The task is to write the Redux selectors required to retrieve the data likely to be required by components that will build the sidebar navigation in Calypso.

These could be written in isolation (with tests!) or alongside a very simple "mega" component which utilises the selectors in order to build a very basic implementation of the navigation. Indeed writing alongside a component may be necessary in order to fully stress test the selectors and to ensure we are getting the "right" data.

Tasks

• Write selector to retrieve "all" nav menu items.
• Write any other selectors required to build and render a simple Nav build from API data.
• Write basic unit tests if this is helpful and aids in development.