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/dev/index.tsx

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2021 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import React from 'react';
+import { createDevApp } from '@backstage/dev-utils';
+import { codeCoveragePlugin, CodeCoveragePage } from '../src/plugin';
+
+createDevApp()
+  .registerPlugin(codeCoveragePlugin)
+  .addPage({
+    element: <CodeCoveragePage />,
+    title: 'Root Page',
+  })
+  .render();