modem: modem_cellular: consolidated user pipe data in to single macro

Introduced the `MODEM_CELLULAR_DEFINE_AND_INIT_USER_PIPES` macro to
consolidate multiple user pipe data definitions into a single macro call.

This will reduce the number of redundant lines in the code, aiming for
improved maintainability.

Signed-off-by: Pisit Sawangvonganan <[email protected]>

Author: ndrs-pst