Feature: HydraDX dynamic fee currency

[bvotteler]

HydraDX has a feature to set a different extrinsics fee currency which then is used for future extrinsic submissions.
Our xcm lib does not take that into account and assumes the fee currency will be the native currency (HDX).
Therefore, the xcm bridge can run into errors when the user doesn't have enough fee currency available to submit the xcm extrinsic (but has enough HDX if the fees were still in HDX).

Therefore, we want to add:

• an adapter or balance adapter method that gets the fee currency. The default implementation returns the native currency, and
• HydraDX's implementation discovers and returns the actual set currency, and
• the fee estimation implementation should be modified to utilize those methods to provide the estimated extrinsic submission fee correctly.