Mesh 2104/add mediator

[HenriqueNogara]

changelog

new features

• Allows instructions and assets to have mediators;

  new external API

  • Adds the following storage to the asset pallet: MandatoryMediators;
  • Adds the following error to the asset pallet: NumberOfAssetMediatorsExceeded;
  • Adds the following extrinsics to the asset pallet: add_mandatory_mediators, remove_mandatory_mediators;
  • Adds the following storage to the settlement pallet: InstructionMediatorsAffirmations;
  • Adds the following error to the settlement pallet: CallerIsNotAMediator, InvalidExpiryDate, MediatorAffirmationExpired;
• Adds the following extrinsics to the settlement pallet: add_instruction_with_mediators, affirm_instruction_as_mediator, withdraw_affirmation_as_mediator, add_and_affirm_with_mediators, reject_instruction_as_mediator;

new events

• Adds the following events to the asset pallet: SetAssetMediators, AssetMediatorsRemoved;
• Adds the following events to the settlement pallet: MediatorAffirmationReceived, MediatorAffirmationWithdrawn;