add tx-limit feature to limit to_private transfer

[zqhxuyuan]

• Choice1: add a new pallet to set asset config
  - Choice2: asset limit is kind of asset metadata, we can add a new field to asset-metadata
• intergation with pallet-manta-pay
  • pallet-manta-pay use FungibleLedger trait, add another ensure_valid(asset_id, amount) to make sure tx is valid
    • amount is allowed when asset_id has limitation
    • or asset_id has no limitation.
  • add another trait and let pallet-manta-pay directly use TransactionLimit as implementation

[Garandor]

• • Choice2: asset limit is kind of asset metadata, we can add a new field to asset-metadata

Why did you get rid of Choice 2? I don't see a reason why it wouldnt work. We can adjust it using the update_asset_metadata extrinsic, but we'd have to add it to #814 before the RT goes live.