api,modem: new 'GetCellInfo()' method

This new method allows querying the modem for information about the current serving cell(s) as well as any other neighboring cell that may be found.

The information for the cells is given in an array of dictionaries, where each element of the array represents a single cell.

Each cell type has a different set of properties that may be given in the dictionary, and some of those properties in each type are also applicable under certain conditions (e.g. only applicable to the cell if it's a 'serving' cell instead of 'neighboring').

The API documentation explains in detail what is expected in each case.