net: openthread: Convert API worker thread to a work queue

Direct openthread API usage requires explicit locking,
which is also used internally.
Exposing a work queue through the openthread context allows
work to be submitted without the need to block other threads.

In particular with CONFIG_OPENTHREAD_MANUAL_START, application
logic can offload work which otherwise would need to wait for
the lock to become available.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>