…ar C API library

Summary:
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This commit adds complete support for the XBee 3 Cellular module in the existing modular C-based XBee API framework, designed with subclassed architecture similar to XBee LR (LoRaWAN). The following major additions and improvements have been made:

Features:
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- Implemented `xbee_cellular.c` and `xbee_cellular.h` with full subclass virtual table and high-level APIs:
  - Cellular-specific initialization, connect, disconnect, sendData, reset, and configuration hooks.
  - Introduced `XBeeCellularConfigure()` for APN/SIM/Carrier setup.
  - Defined all necessary AT command enums (e.g., `AT_PN`, `AT_AN`, `AT_CP`, `AT_SD`).
  - Implemented IPv4 TX frame sending using `XBEE_API_TYPE_CELLULAR_TX_IPV4`.

- Added `xbee_cellular_example.c`:
  - Demonstrates setup, initialization, connection, and periodic data send using XBee Cellular.
  - Follows same structure as the existing LR example.
  - Implements stubs for receive/send callbacks.

- Updated `xbee_api_frames.h` and `xbee_at_cmds.h`:
  - Defined new AT commands and API frame types for cellular.
  - Updated documentation blocks for full consistency.

- Extended core base logic in `xbee.c`:
  - Added `XBeeConfigure()` dispatch to subclass `configure()` method.