Re-Add CodeCov

[blaginin]

Which issue does this PR close?

Related to #15155 (comment)

Rationale for this change

We have quite a lot of code that isn't covered in tests. And it's actually easy to miss covering some case when you submit a big PR

What changes are included in this PR?

Now, when a new PR introduces untested code, a warning message will be posted. Example: here.

Coverage data is aggregated from:

• linux-test
• linux-test-datafusion-cli
• sqllogictest-postgres
• macos-aarch64
• test-datafusion-pyarrow

Excluded from coverage:

• datafusion-examples — not technically tests.
• verify-benchmark-results — IMO we shouldn't rely on them for functionality tests, those should be covered on the unit test level
• doctests — because they requite nightly rust and I couldn't (yet) make it work in the CI (although will try more)

There are ways to make checks even more annoying (e.g. putting ❌ in CI checks). I want to do this later, but first, I want to make sure we're measuring coverage properly and others are okay with it. If that’s the case, we’ll tighten CI checks in future PRs.

Absolute coverage example: https://app.codecov.io/gh/blaginin/datafusion/pull/4/tree

Are there any user-facing changes?

No.

[blaginin]

To make this work, we'll need to add CODECOV_TOKEN and add the app to the repo (may already be there since we used it before)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Additional details and impacted files

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Omega359]

I got a lot of pushback on code coverage when I brought it up, fyi. Here is a report I had done with a manual run.