fix(xcm): Change weight.ref_time() casting from u128 to u64

[crStiv]

Convert weight.ref_time() casting from u128 to u64 as appropriate for the Weight implementation.

Changed in:

• primitives/src/xcm/mod.rs (buy_weight and refund_weight methods)
• primitives/src/xcm/tests.rs (execution_fee function)
• pallets/xc-asset-config/src/lib.rs (weight_to_fee method)

This fixes a potential overflow issue when working with large weight values.

[Dinonard]

Thank you but we don't need this right now.