[Bug]: Validate the coordinator proposed bitcoin/stacks fees

[matteojug]

Bug - Validate the coordinator proposed bitcoin/stacks fees

1. Description

The fee used for bitcoin/stacks transaction is decided by the coordinator, and the signers blindly accept it. Instead, they should validate that the proposed one is "reasonable" given their own view of bitcoin/stacks.

2. Why It's Necessary

Without validation, a coordinator can force the users to spend more than required in fees (still, within their specified max_fee) or waste the STX funds allocated for the stacks transactions fees.

[AshtonStephens]

I'm going to pick this one up. The goal is to have a procedure that signers can follow to prevent themselves from approving transaction that uses far too high of a fee, but is also resilient against bad stacks fees being reported from the estimator.

Here's the procedure I'm planning to implement:

Variables

• the config will specify both a maximum and minimum sats / vbyte and stx fee.
• a constant will specify the % more than the signer's estimate that the coordinator's estimate can be for the signer to approve the transaction

Procedure

1. If our fee estimate is outside of specified bounds then use the coordinator's fee suggestion unless coordinator suggestion is also out of bounds, in which case reject. This will make the signer resilient to its own STX or BTC fee estimate being bad
2. If our fee estimate is inside of specified bounds:
   1. If the coordinator suggests a fee that exceeds our maximum then reject
   2. If the coordinator suggests a fee more than constant x% above our estimate then reject

[AshtonStephens]

It's not strictly necessary to do this for Bitcoin as well as Stacks - but having different ways of specifying these things for Stacks and Bitcoin when the problem is the same will make the configuration more confusing and require more explanation to the people running the signer - and therefore introduce more opportunities for user error.

I'd rather allow more options and have sensible defaults than have specialized ways of defining how to handle the same problem between the Bitcoin and Stacks blockchains.