xcm-builder: PayOverXcm supports fallible convertors for asset kind and...

xcm-builder: PayOverXcm supports fallible convertors for asset kind and beneficiary conversion  (#1572)

`PayOverXcm` type accepts two converters to transform the `AssetKind`
and `Beneficiary` parameter types into recognized `xcm` types. In this
PR, we've modified the bounds for these converters, transitioning from
`Convert` to `TryConvert`.

One such use case for this adjustment is when dealing with versioned xcm
types for `AssetKind` and `Beneficiary`. These types might be not
convertible to the latest xcm version, hence the need for fallible
conversion.

This changes required for
https://github.com/paritytech/polkadot-sdk/pull/1333