RFC: API Change: mgmt: mcumgr: Replace mgmt_ctxt struct with smp_streamer

McCrae, Jamie

I have submitted an RFC for a stable API change in MCUmger to drop usage of the `mgmt_ctx` struct and replace it with the `smp_streamer` struct, this removes a layer of indirection and allows command handlers to access the SMP transport object. RFC link: https://github.com/zephyrproject-rtos/zephyr/issues/52021
This would affect applications applications registering custom MCUmgr command handlers, 1-3 lines per command handler would need to be updated.