Add metedata to the report file

[rgreinho]

This patch adds a new metadata section to the report file being
generated by chain-bench.

This initial version adds the generation date, and the check statistics
which is useful to compare different scans and their evolutions over
time.

Fixes #38

[CLAassistant]

All committers have signed the CLA.

[rgreinho]

The report file would now look like this (the results part in the snippet was shortened for readability):

{
  "Metadata": {
    "Date": "2022-06-28T13:40:36-05:00",
    "Statistics": {
      "Passed": 5,
      "Failed": 23,
      "Unknown": 8,
      "Total": 36
    }
  },
  "Results": [
    {
      "ID": "1.1.3",
      "Name": "Ensure any change to code receives approval of two strongly authenticated users",
      "Descrition": "Ensure that every code change is reviewed and approved by two authorized contributors who are strongly authenticated.",
      "Remediation": "An organization can protect specific code branches — for example, the \"main\" branch which often is the version deployed to production — by setting protection rules. These rules secure your code repository from unwanted or unauthorized changes. You may set requirements for any code change to that branch, and thus specify a minimum number of reviewers required to approve a change.",
      "Result": "Failed",
      "Reason": "",
      "Url": "https://avd.aquasec.com/compliance/softwaresupplychain/cis-1.0/cis-1.0-sourcecode/1.1"
    }
  ]
}