Add number of violations reported by Sonar to the stats output

[khesoem]

Currently, if Sorald does not fix any violations, we do not report the total number of violations. We can add an attribute to the json file to note how many violations are detected by Sonar.

For example, the following stats file does not show how many violations exist in the code:

{
    "repairs": [],
    "executionInfo": {
        "soraldVersion": "LOCAL",
        "javaVersion": "11.0.11",
        "originalArgs": [
            "repair",
            "--source",
            "zendesk-java-client",
            "--rule-key",
            "2272",
            "--stats-output-file",
            "output-2021-12-10-15-02-12/zendesk-java-client_2272_sorald_repair_stats.json"
        ]
    },
    "totalTimeMs": 24897,
    "repairTimeMs": 0,
    "startTimeMs": 1639149077672,
    "crashes": [],
    "endTimeMs": 1639149102569,
    "parseTimeMs": 0
}

Adding a sonarDetectedViolations at the root level should solve this.