Ability to query correct Xcm destination weights

[jacogr]

Currently on the asset teleports, we need to specify the destination weight. (Effectively calculated from the BaseXcmWeight) From a use perspective, this is currently hardcoded and already has some issues between different chains (e.g. Westend vs Kusama) since the parameters are different.

It would be optimal and much less error prone if we were to expose an rpc along the lines of queryXcmWeight(Xcm) -> Weight (copy-and-pasted from Gav's suggestion) that can be used to estimate this value. Since we pass in the encoded Xcm call, we can use it to estimate any Xcm message, not just the one I wish to use it for in the short term.

This can be used for -

• the dest weight
• calculations with the WeightToFee (the above RPC would, in a perfect world, already return this)

Obviously getting this value 100% correct is quite important here, getting it wrong can lead to sending assets via teleport into a black hole.