refactor: split core library functions for customers

[Chethan-rao]

Type of Change

• [ ] Bugfix
• [ ] New feature
• [ ] Enhancement
• [x] Refactoring
• [ ] Dependency updates
• [ ] Documentation
• [ ] CI/CD

Description

Currently our file structure looks like this

|--crates
|  |--router/src
|  |  |--core
|  |  |  |--customers.rs

This PR includes changed for inverting it to look like this

|--crates
|  |--router/src
|  |  |--customer
|  |  |  |--core.rs

Additional Changes

• [ ] This PR modifies the API contract
• [ ] This PR modifies the database schema
• [ ] This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

Checklist

• [x] I formatted the code cargo +nightly fmt --all
• [x] I addressed lints thrown by cargo clippy
• [x] I reviewed submitted code
• [ ] I added unit tests for my changes where possible
• [ ] I added a CHANGELOG entry if applicable