fix: Switch from domainName to domainID

[freddyli7]

Description

• Switch from domainName to domainID in gas price fetching job
• Removed impl of Polygonscan and merge gas oracle impl
• config structure changes: introducing domain list with domainID into config
• api service apikey in env var now is dynamic: gas price api service key format <SOURCE>_API_KEY_<DOMAIN_ID>, e.g: ETHERSCAN_API_KEY_0 if domain 0 is using etherscan to fetch gas price
• conversion rate apikey in env var now is dynamic: conversion rate api service key format <SOURCE>_API_KEY. e.g: COINMARKETCAP_API_KEY

Related Issue Or Context

Closes: #24 #26

How Has This Been Tested? Testing details.

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