samples: net: sockets: echo_client: Fix userspace crash

k_work_* APIs cannot be used from user threads, hence it has to be
replaced with some other solution to retain userspace functionality in
the sample.

This commit reworks the sample as follows:
  * Replace delayed work items with combination of k_timer and a
    dedicated thread to send UDP packets. k_poll_signal is used for
    communication between k_timer callback and UDP thread.
  * As kernel objects should not be placed in a memory modifiable from
    user threads, declare a separate structure for them, and link it
    with the client context structure with a pointer.
  * k_timer_init() is not a system call, therefore it has to be called
    from supervisor thread. Therefore, add an additional function to
    initialize UDP, and use it to initialize kernel objects used by the
    UDP module and grant access for the main thread before it becomes an
    user thread. UDP thread inherits permissions from the parent (main
    thread).

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