mgmt/mcumgr/lib: Add OS_MGMT_TASKSTAT_STACK_INFO to mcumgr

The commit adds OS_MGMT_TASKSTAT_STACK_INFO Kconfig option
that allows to skip, when disabled,  "stksz" and "stkuse" reports,
in "taskstat" command responses.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>