[mq pallet] Custom next queue selectors (#6059)

Changes:
- Expose a `force_set_head` function from the `MessageQueue` pallet via
a new trait: `ForceSetHead`. This can be used to force the MQ pallet to
process this queue next.
- The change only exposes an internal function through a trait, no audit
is required.

## Context

For the Asset Hub Migration (AHM) we need a mechanism to prioritize the
inbound upward messages and the inbound downward messages on the AH. To
achieve this, a minimal (and no breaking) change is done to the MQ
pallet in the form of adding the `force_set_head` function.

An example use of how to achieve prioritization is then demonstrated in
`integration_test.rs::AhmPrioritizer`. Normally, all queues are
scheduled round-robin like this:

`| Relay | Para(1) | Para(2) | ... | Relay | ... `

The prioritizer listens to changes to its queue and triggers if either:
- The queue processed in the last block (to keep the general round-robin
scheduling)
- The queue did not process since `n` blocks (to prevent starvation if
there are too many other queues)

In either situation, it schedules the queue for a streak of three
consecutive blocks, such that it would become:

`| Relay | Relay | Relay | Para(1) | Para(2) | ... | Relay | Relay |
Relay | ... `

It basically transforms the round-robin into an elongated round robin.
Although different strategies can be injected into the pallet at
runtime, this one seems to strike a good balance between general service
level and prioritization.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: muharem <ismailov.m.h@gmail.com>