[MLA-803] Add timer metadata to C# and python

[chriselion]

Proposed change(s)

The main goal of this was to add a version string to the timer data, but might as well add some useful metadata while I'm at it.

C#:

    "metadata": {
        "timer_format_version": "0.1.0",
        "start_time_seconds": "1586382781",
        "unity_version": "2018.4.17f1",
        "communication_protocol_version": "0.15.0",
        "package_version": "0.15.0-preview",
        "scene_name": "Basic",
        "end_time_seconds": "1586382784"
    }

python:

    "metadata": {
        "timer_format_version": "0.1.0",
        "start_time_seconds": "1586382867",
        "python_version": "3.7.6 (default, Dec 30 2019, 19:38:28) \n[Clang 11.0.0 (clang-1100.0.33.16)]",
        "mlagents_version": "0.16.0.dev0",
        "mlagents_envs_version": "0.16.0.dev0",
        "communication_protocol_version": "0.15.0",
        "tensorflow_version": "2.0.1",
        "end_time_seconds": "1586382879"
    }

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[chriselion]

Since only the root node needs gauges and metadata, I made the RootNode a separate class. Just a memory micro-optimization.