Add codecov.yaml file to enforce test coverage threshold #4

Add + Setup codecov.yaml in project root to enforce test coverage threshold. This will ensure job fails if threshold is not met.

Use docs from https://docs.codecov.com/docs/codecov-yaml

Acceptance Criteria:

• Threshold should be set to 80%
• Job fails if below threshold
• Job passes if at or above threshold
• Threshold should be synced for codecov as well as for local testing