Quectel AT command: request and response are not just bytearrays

Looking at the changes introduced in !212 (merged), I can see how:

• the request is not only an unsized byte array, the first 4 bytes identify the command type, 0 being for AT.
• the response is not only an unsized byte array, the first 4 bytes report the status of the response.

We must not have unsized byte arrays as a way for MBIM service definitions to hide the format and details of the messages, so we should not allow this type of change.

We should:

• Update the request definition to add a "guint32" field before the request bytearray, including a "public-format" and a new enum identifying the supported values.
• Update the response definition to add a "guint32" field before the response bytearray, including a "public-format" and a new enum identifying the supported values.

Please work on this issue ASAP, because in order to keep API stability we should not have this update introduced after the next stable release.