Use codecov.io to track code coverage

[roypat]

Currently, we track code coverage targets using a coverage file in the repository root, which needs to be manually updated every time code coverage changes (both improvements and reductions). This issue aims to start a discussion around stopping to do that, and instead use codecov.io to track our coverage for us. We've been using it over at Firecracker for about a year now.

Pros:

• No more manual tracking of coverage numbers in the repository
• Graphical coverage reports (see for example https://app.codecov.io/gh/firecracker-microvm/firecracker)
• On each PR, a summary report showing which changes lines in the PR are covered/uncovered (see for example Hugepages support for Firecracker firecracker-microvm/firecracker#4360 (comment))

Cons:

• No more CI blocking if coverage is lowered by a PR (although realistically in the past we always just "overrode" those anyway by updating the file, so not sure if this will really be much of a change)

[roypat]

Update from community meeting on 2025/03/03: We want to experiment with codecov.io - @roypat will work on this (although no set timeframe yet). We'll implement a sort of opt-in mechanism in rust-vmm-ci so that we can experiment with it on specific crates to see if we like it. First steps will be adding the codecov app to our github organization. Then we'll need to update test_coverage.py to submit coverage to codecov.io instead of checking the local .json file if the repo has opted into codecov.