Make AssetHub as DOT reserve

[dmoka]

We need this as a part of issue https://github.com/galacticcouncil/hydration-node/issues/857

What we want for now:

• DOT can be sent to hdyra.
• And DOT can be sent to AH from hydra.

The main use case is AssetHub is the main hub for every assets, so there will be many transacitons where we want to use DOT as fee.

TODO: once the draft PR is ready, we should adjust/close PR https://github.com/galacticcouncil/hydration-node/pull/885