mgmt: mcumgr: grp: settings_mgmt: Add capability to save setting by key

Extend the save command to allow passing name/val pair to save a
subtree or individual setting to persistent storage.
Resolves zephyrproject-rtos#90407

Co-authored-by: Jamie <40387179+nordicjm@users.noreply.github.com>
Signed-off-by: Rami Saad <rami.saad@morgansolar.com>
3 files changed