refactor(configs): make TOML file an optional source of application configuration

[SanchithHegde]

Type of Change

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

Description

This PR refactors the application configuration deserialization code to make the TOML file as an optional source of application configuration.

Motivation and Context

The TOML file was made as a required source of application configuration when support for deserializing application configuration from environment variables was absent. Since we have support for environment variables now, we can make the TOML file as an optional source.

How did you test it?

Ran it locally without config/development.toml, binary does not complain about missing development.toml file.

Checklist

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