Add additional diagnostics

[pbolduc]

CONFIG_ASYNC_TCP_DIAGNOSTICS is an opt-in feature. If CONFIG_ASYNC_TCP_DIAGNOSTICS, the following tracking and data will not be compiled in. The library will expose a way to request this data. Applications can get the data and use it how it needs.

• [x] Async task stack highwater mark
• [ ] Connection failure count
• [ ] Memory allocation failure count
• [ ] Async queue current item count
• [ ] Async queue send failure count