drivers/modem/modem_cmd_handler: Update API

There is currently not a clear separation between
user configuration and internal context when using the
modem_cmd_handler library. This update adds a clear
separation, placing user configuration in a seperate
struct passed to modem_cmd_handler_init alongside the
internal context modem_cmd_handler_data.

There is also a lack of documentation of the user
configurations, these have been added to the new config
struct.

The new API function modem_cmd_handler_process has been
added to remove the need for the user to directly access
the process member of the internal context. This ensures
that the user is not encouraged to access any internal
context members.

Some whitespace errors exist in the modem_cmd_handler.c
file, these are outside of the scope of this PR. These
can be addressed in a later PR as they are not functional
changes.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>