net: coap: Rework pending retransmission logic

Introduce retransmission counter to the coap_pending structure. This
allows to simplify the retransmission logic and allows to keep track of
the number of remaining retranmissions.

Additionally, extend the `coap_pending_init()` function with `retries`
parameter, which allows to set the retransmission count individually for
each confirmable transaction.

Fixes #28117

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>