` XcmContext` to `buy_weight / refund_weight` (#7563)

* added XcmContext to WeightTrader trait

* cargo fmt

* make xcm context optional

* make compile

* fix compile

* `XcmContext` to `buy_weight / refund_weight`

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Co-authored-by: Patricio Napoli <[email protected]>
Co-authored-by: Giles Cope <[email protected]>
Co-authored-by: parity-processbot <>