Bluetooth: conn: Allocate TX context JIT

`bt_conn_send_cb` used to allocate a TX context (K_FOREVER).
Instead, we now put the context in the userdata of the buffer.

This means that now this fn will never block and always succeed since the
tx_queue is a FIFO (infinite size). It just puts the buf on the queue.

The metadata is stored safely in there until we have acquired all the
necessary resources to send it to the controller without failing: TX
context and controller buffer.

I.e. when `bt_conn_process_tx` is called, that's when a TX context is
try-allocated and the contents of `buf->userdata` is moved into it.
The buffer is now ready to be sent to the lower layer.

`bt_conn_process_tx` will return -EWOULDBLOCK if it's not able to acquire a
TX context, this PR modifies `bt_conn_prepare_events` to respond to this by
also waiting on the TX context pool.

Unfortunately, this increases the required userdata size for any buffers
handed to `bt_conn_send_cb`. This will be fixed in a later commit.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>