Refresh of async-dynamic-cookies

Similar to `cookies()`, `headers()` provides access to an underlying Request in Next.js. To be optimally compatible with dynamicIO we need request access to be implemented through async APIs and have udpated this function to return a promise of Headers rather than Headers directly. To facilitate migration this API will augment the Promise with properties that allow direct access to headers. In a future version of Next.js we will remove the exotic nature of this Promise and all access will require awaiting the result.