feat: extend fee calculation formula

[nmlinaric]

Description

Extends the fee calculation formula with the option to charge on top of execution fee a protocol fee.
Protocol fee can be a fixed amount of native token or a percentage of the execution cost.

Related Issue Or Context

Closes: #242

How Has This Been Tested? Testing details.

unit test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in chainbridge-docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot