fix: handle fetching conversion rate for same token

[MakMuftic]

This is a temporary addition to handle Goerli<>Sepolia case.

This case will be covered when we start refactoring these conversion pair definitions #34

Description

Related Issue Or Context

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