Increase threshold for failed code coverage checks

[alyssadai]

Let's use https://about.codecov.io/ consistently across repos and also increase threshold for failed checks.

• Add code coverage using Codecov tools#7
• https://github.com/neurobagel/api
• Measure and show code coverage query-tool#62
• https://github.com/neurobagel/federation-api
• https://github.com/neurobagel/bagel-cli

At the moment, our code coverage checks (e.g., via coveralls) fail when coverage decreases by any amount. For example, see neurobagel/api@a93d476 which failed the coverage check due a 0.7% decrease.

To avoid possibly superfluous tests just for the sake of ensuring 0% coverage decrease, I propose we bump up the threshold so that coveralls / codecov only complains when coverage has decreased 1% or more.

[surchs]

Could we make it relative to the PR rather than the codebase? I.e. we allow up to X% (maybe 20%?) of new or changed lines in a PR to be uncovered? % of the whole codebase is a bit harder to judge for me