Add BG96 support direct push socket

[chinglee-iot]

Add BG96 support direct push socket.

This commit provides an example of input buffer callback in this PR FreeRTOS/FreeRTOS-Cellular-Interface#137.

Socket data is returned in the URC with direct access mode. To support this feature, the port make use of the input buffer callback to handle the binary data in URC.
In the following example, the socket data "test" will be stored in the buffer in module context. User of this port can retrieve the data with Cellular_SocketRecv later.

+QIURC: "recv",0,4
test

Compare to buffer access mode, the port will allocate an extra buffer per socket to store the socket data. This useful if developers want to configure the buffer size in MCU instead of relying on the buffer size in modem.

Description

• This mode doesn't make use of the buffer in cellular modem. The socket data received is directly pushed through URC code. Extra buffer for socket will be required if enabled. Default buffer size is 2048 and can be configured with CELLULAR_BG96_DIRECT_PUSH_SOCKET_BUFFER_SIZE.
• Define CELLULAR_BG96_SUPPPORT_DIRECT_PUSH_SOCKET in cellular_config.h to enable this function. Default is disabled.

Test Steps

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

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