Show Code Coverage badge in README.md

[wba2hi]

Describe the solution you'd like
Show a badge with the current code coverage for the main branch in README.md. README.md contains static content, therefore it is required to have a static URL which points to the "latest" code coverage image.

Solution ideas

• deploy the badge on github-pages: https://hackernoon.com/adding-test-coverage-badge-on-github-without-using-third-party-services -> if we want to use gh-pages in the future this adds a lot of noise (each merge to main -> update badge commit) and should therefore be avoided
• deploy the badge to gist and link it from there: https://github.com/GaelGirodon/ci-badges-action
• ... other approaches

Deploying to gist would be the most favorable approach, but we need a responsible person to create the gist and provide the token as secrets to the corresponding organizations / repositories. The eclipse support can probably help here by either creating or probably even re-using an existing gist api token.

Additional context

• a no cost service should be used => services which provides "free licenses" for Open Source Projects must NOT be used (e.g. codeclimate, codecov, codacy, coveralls)
• GitHub bring-your-own-badge action must NOT be used without clarification because it stores the data on their own server.