Add helper types for HandlerFunctions

[daogrady]

Fixes cap-js/cds-typer#423

sample use:

import { HandlerFunction } from '@cap-js/cds-types'
import { myAction } from '#cds-models/…'

// …

srv.on(myAction, onMyAction)

function onMyAction (req: HandlerFunction<typeof myAction>['parameters']['req']: HandlerFunction<typeof myAction>['returns']> {

// or

type ActionType = HandlerFunction<typeof myAction>

function onMyAction2 (req: ActionType['parameters']['req']): ActionType['returns']

  // …
}