Add code coverage CI.

[maryla-uc]

Add a cmake option.
Document in readme.

After this is merged I will delete the seriously outdated instructions in the wiki https://github.com/AOMediaCodec/libavif/wiki/Coverage

[maryla-uc]

Note the CI is not strictly required, but it makes sure the coverage stuff actually works, and uploads the coverage report as a github artifact that can be downloaded e.g. here https://github.com/AOMediaCodec/libavif/actions/runs/12584719944?pr=2541

[vrabaud]

We could also upload those to https://app.codecov.io/ . and have a little badge on top of our README.

[maryla-uc]

Indeed, that looks nice! I think it can be done as a second step (in another PR).

[maryla-uc]

I tried setting up codecov.io but I think you need to be a member of the AOMediaCodec organization?
Basically you need to log into codecov.io with your github account, and it should show a list of github repos you can configure. But in my case it only shows my own fork of libavif (among other forks I have), not AOMediaCodec/libavif. @wantehchang would you have the necessary permissions to set it up? See their quick start guide: https://docs.codecov.com/docs/quick-start