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)