chore: Add code coverage reporting via CodeCov

[scouten-adobe]

As a user of this crate, I'd like to know how well this code is covered by its own self tests.

I'm certainly open to alternatives to CodeCov, but I'd like to see something that shows active development of tests designed to assert the overall reliability of this code and show coverage of edge cases.

I've commented out two pieces that will require intervention from the SpruceID team:

1. Support for WASM code coverage is commented out until WASM builds are viable. (See #544.)
2. Reliability of the upload service will improve once a CODE_COV token is provided. This has to be done by the repo owner.