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Allow average price source to be used with references (#1215)

The struct AveragePriceSource forces ownership semantics onto the user.
We extract the average computation out of the struct so that it can be
used with references.
This is going to be useful for the price estimator where it won't be possible for AveragePriceSource to own the pricegraph based price source but we nonetheless want to calculate the average by reusing the existing code.

### Test Plan
No logic change, existing tests still work.

---

I also tried to make a more advanced version that makes the average price source generic over whether it owns the price source or has references. 

```rust
struct AveragePriceSource_<T0, T1: ?Sized, T2> {
    t: T0,
    _t1: PhantomData<T1>,
    _t2: PhantomData<T2>,
}

impl<'a, T0, T1, T2> AveragePriceSource_<T0, T1, T2>
where
    T0: AsRef<T1>,
    T1: 'a + ?Sized,
    &'a T1: IntoIterator<Item = T2>,
    T2: AsRef<dyn PriceSource + 'a>,
{
    fn new(t: T0) -> Self {
        Self {
            t,
            _t1: PhantomData,
            _t2: PhantomData,
        }
    }

    async fn get_prices(&'a self, tokens: &[TokenId]) -> Result<HashMap<TokenId, NonZeroU128>> {
        // For brevity we return the result of the first price source but in the real code we would average.
        let t1: &'a T1 = self.t.as_ref();
        let mut iter = t1.into_iter();
        let first = iter.next().unwrap();
        let source: &dyn PriceSource = first.as_ref();
        source.get_prices(tokens).await
    }
}

#[test]
fn compiles() {
    type BoxDyn = Box<dyn PriceSource>;

    let owned: Vec<BoxDyn> = Vec::new();
    let _ = AveragePriceSource_::<_, [BoxDyn], _>::new(owned).get_prices(&[]);

    let slice: &[BoxDyn] = &[];
    let _ = AveragePriceSource_::<_, [BoxDyn], _>::new(slice).get_prices(&[]);

    let slice_of_references: &[&BoxDyn] = &[];
    let _ = AveragePriceSource_::<_, [&BoxDyn], _>::new(s... (continued)

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