[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.