net: lwm2m: use server record to set default observe notify timing

Server records contain the default PMIN and PMAX settings for how
often we can send observe notifications.  We are currently using
arbitrary defaults which cannot be changed when compiled or
during runtime.

Let's add Kconfig settings for the default settings to use and
also lookup the current values in the active server record when
an observe is added.

The actual PMIN/PMAX values can still be set via WRITE_ATTRIBUTE
operation.

Signed-off-by: Michael Scott <mike@foundries.io>