mgmt/mcumgr/lib: Reduce taskstat to Zephyr supported statistics

The commit adds CONFIG_OS_MGMT_TASKSTAT_ONLY_SUPPORTED_STATS
Kconfig option that disables code filling in task statistics, for
mcumgr command `taskstat`, that are not collected or supported by
Zephyr.
Setting this option to y will skip following statistics in response:
"runtime", "cswcnt", "last_checkin" and "next_checkin".

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