Add Clirr Check

## Expected Behavior

This library should list breaking changes such that `mvn clirr:check` passes.

This can be achieved by creating a new file:

```
clirr-ignored-differences.xml
```

And making sure that we do not skip the clirr check by removing this line (no skip in `.kokoro/stage.sh`):

```sh
-Dclirr.skip
```

## Examples:

- File: https://github.com/googleapis/java-monitoring/blob/master/google-cloud-monitoring/clirr-ignored-differences.xml
- PR: https://github.com/googleapis/java-monitoring/pull/161
- Release notes: https://github.com/googleapis/java-monitoring/releases/tag/v2.0.0

## Actual Behavior

This check is disabled.