Add slippage acceptance for fee's validation

[samuelmanzanera]

Description

Add splipage acceptance for tx's fee validation of 3%

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Unit test

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Neylix]

Look's good, but the same control is missing in the TransactionValidator module.
I suggest to create a function like Fee.valid_variation? to unify both workflow