Create data layer integration for /admin-menu endpoint

[getdave]

Sub Issue of #45435.

This documents the need to (experiment with) creating a data-layer and state implementation for the admin-menu endpoint proposed in D49005-code.

Tasks

• Create new admin-menu directory in client/state/data-layer/wpcom.
• Define Redux action namespace (eg: ADMIN_MENU_).
• Register handler module for all menu items (registerHandlers).
• Wire up basic WPCom HTTP request using dispatchRequest and logging from the various response actions creators.
• Write data transformer to transform the API response into consumble data.

Following this, the next step will be to create a state tree for admin menus with actions, reducers and selectors to make the data available to the UI.