Bluetooth: host: Change TX pattern (push -> pull)

The current TX pattern in the host is to try to push a buffer through all
the layers up until it is ingested by the controller.

Since sending can fail at any layer, we need error-handling and separate
retry logic on pretty much all layers. That logic obscures the "happy path"
for people trying ot understand the code.

This commit inverts the control, in a way that doesn't require changing the
host or HCI driver API (yet):

Layers don't send buffers synchronously, they instead put their buffer in a
private queue of their own and raise a TX flag on the lower layer. Think of
it as a `READY` interrupt line that has to be serviced by the lower layer.

Sending is now non-blocking, rate depends on the size of buffer pools.

There is a single TX processing function. This can be thought as the
Interrupt Service Routine that will handle the `READY` interrupt from the
layers above.

That `tx_processor()` will then attempt to allocate enough resources in
order to send the buffer through to the controller. This allocation logic
does not block.

After acquiring all the resources, the TX processor will attempt to pull
data from the upper layer. The upper layer has to figure out which buffer
to pass to the controller. This is a good spot to put scheduling or QoS
logic in the upper layer.

Notes:

- user-facing API for tuning QoS will be implemented in a future patch

- this scheme could (and probably will) be extended to upper layers (e.g.
  ATT, L2CAP CoC segmentation).

- this patch removes the `pending_no_cb()` memory optimization for
  clarity/correctness. It might get re-implemented after a stabilization
  period. Hopefully with more documentation.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
Co-authored-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>

tests/bsim/bluetooth/ll/bis/src/main.c

@@ -153,7 +153,8 @@ bool ll_data_path_sink_create(uint16_t handle, struct ll_iso_datapath *datapath,
 
 #define BUF_ALLOC_TIMEOUT_MS (30) /* milliseconds */
 NET_BUF_POOL_FIXED_DEFINE(tx_pool, CONFIG_BT_ISO_TX_BUF_COUNT,
-			  BT_ISO_SDU_BUF_SIZE(CONFIG_BT_ISO_TX_MTU), 8, NULL);
+			  BT_ISO_SDU_BUF_SIZE(CONFIG_BT_ISO_TX_MTU),
+			  CONFIG_BT_CONN_TX_USER_DATA_SIZE, NULL);
 
 static struct k_work_delayable iso_send_work;