feat: add native token handler and adapter

[nmlinaric]

Description

Adds adapter and native token handler (modifier erc20 handler) for native token transfers (intended to be used for BTC<>EVM)

Related Issue Or Context

Closes: #231

How Has This Been Tested? Testing details.

unit tests

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

[lastperson]

I like the code, perhaps Adapter could use the docs,