samples: add userspace producer/consumer sample

We try to demonstrate some concepts for user mode:

- Multiple logical applications, each with their own memory
  domain
- Creation of a sys_mem_pool and assignment to a memory
  partition
- Use of APIs like k_queue_alloc_append() which require
  thread resource pools to be configured
- Management of permissions for kernel objects and drivers
- Show how application-specific system calls are defined
- Show IPC between ISR and application (using k_msgq) and
  application-to-application IPC (using k_queue)

Fixes: #14683

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>