Improve DropAssets to support assets below ExistentialDeposit into treasury

[zqhxuyuan]

we may want to allow assets below ExistentialDeposit directly into treasury account in the DropAssets case instead of go to AssetsTrap storage. As there're already an TakeRevenue trait in xcm-builder, we could just use this trait to do our deposit logic. another problem to be consideration is how to get ExistentialDeposit for different asset, the simple solution is just passing constant map.

By default in the relaychain side, the configuration could be (). in the parachain side, parachain can set different ExistentialDeposit value for different asset just like what we did in acala, and we can deposit those asset into treasury account too.

here is a quick prototype implementation. if it's reasonable, I can create an PR on polkadot codebase. @xlc @KiChjang

there're some rude code in this quick implementation needs to more discuss:

• should we move TakeRevenue to xcm-executor/traits?
• the get_by_key may be replaced if there're some existing utility method.

[zqhxuyuan]

turn out we could implements DropAssets in our own runtime. close it.