Create code-coverage plugin

In order to use this plugin, you must set the
`backstage.io/code-coverage` annotation on your entity.

```yaml
backstage.io/code-coverage: enabled
```

There's a feature to only include files that are in SCM in the coverage
report, this is helpful to not count generated files for example. To
enable this set the `backstage.io/code-coverage` annotation to
`scm-only`.

```yaml
backstage.io/code-coverage: scm-only
```

The backend plugin provides API endpoints for submitting code-coverage
reports. Currently jacoco and cobertura are supported. These reports
are normalized to a json format that is stored in the database.

```json
// curl -X POST -H "Content-Type:text/xml" -d @cobertura.xml "localhost:7000/api/code-coverage/Component/default/entity-name?coverageType=cobertura"
{
    "links": [
        {
            "href": "http://localhost:7000/api/code-coverage/Component/default/entity-name",
            "rel": "coverage"
        }
    ]
}
```

It also provides some additional API endpoints:
* Viewing the latest report
* Viewing a more condensed history of code coverage values
* Retrieving file contents from source-control, used by the UI

Provides a graph of code coverage change over time, as well as a file
view where you can see the highlighted lines.

Co-authored-by: nissayeva <natashaaay@gmail.com>
Signed-off-by: alde <r.dybeck@gmail.com>
Signed-off-by: Fredrik Adelöw <freben@gmail.com>

plugins/code-coverage/.eslintrc.js

@@ -0,0 +1,3 @@
+module.exports = {
+  extends: [require.resolve('@backstage/cli/config/eslint')],
+};