Implement FreeRTOS Cellular Library API with cellular module specific AT commands.

Create FreeRTOS Cellular Library APIs implemented with module specific AT commands with cellular_common.h and cellular_at_core.h
AT command related data structure
- CellularAtReq_t : AT command request data structure
- CellularAtDataReq_t : Data send data structure
Send AT command APIs
- _Cellular_AtcmdRequestWithCallback : Default timeout
- _Cellular_TimeoutAtcmdRequestWithCallback : Custom timeout
Send AT command APIs for data receive ( E.X socket data receive )
- _Cellular_TimeoutAtcmdDataRecvRequestWithCallback
Send AT command APIS for data send ( E.X socket data send )
- _Cellular_TimeoutAtcmdDataSendRequestWithCallback : Basic data send API
- _Cellular_AtcmdDataSend : Prefix callback function to indicate data mode start
- _Cellular_TimeoutAtcmdDataSendSuccessToken : Extra success token to indicate AT commnad success