Allow all custom fee collectors to be exempt

[tinker-michaelj]

HIP-18 (Custom Hedera Token Service Fees) did not stipulate any exemptions for custom fees. The final implementation spec carved out two exemptions,

There are two exemptions from the charging policy. First, token treasury accounts are never charged custom fees. Second, a fee collector account is not charged any fractional fees for which it is the collector.

So the network still charges a fee collector custom fees for which it is not the collector. This can be cumbersome for fungible tokens with multiple fee collectors who exchange units of the token.

We propose to add a field to the CustomFee protobuf message so a custom fee can be created with unconditional exemptions for all fee collectors in the owning token type.

message CustomFee {
  ...
  AccountID fee_collector_account_id = 3;
  bool all_collectors_are_exempt = 4;
}

By default exempt_all_collectors = false, and the current implementation's behavior will be preserved. But suppose token type 0.0.T has two custom fees F1 and F2 with respective collectors 0.0.A and 0.0.B. If F1 was created with all_collectors_are_exempt = true, then when 0.0.B sends units of 0.0.T to 0.0.A, it will not pay fee F1.

[Cooper-Kunz]

This makes sense @tinker-michaelj - can anyone / any key update this value?

[tinker-michaelj]

Hi @Cooper-Kunz, sorry I missed this!

The fee schedule must be present to update this; and it must sign the TokenFeeScheduleUpdate.