Support google timestamps, use date-time format in json schema

[chrusty]

This has been resurrected from a closed PR raised by @huttotw.

From his original PR:

The well known type google/protobuf/timestamp.proto will now be represented as:

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "properties": {
        "timestamp": {
            "type": "string",
            "format": "date-time"
        }
    },
    "additionalProperties": true,
    "type": "object"
}

[chrusty]

@mier85 How would this affect you guys? We never had any specific tests for TimeStamp.

[xylo04]

I'm just barely adopting protoc-gen-jsonschema and therefore not a critical stakeholder, but FWIW, I'm very much in favor of this change.

[xylo04]

I was mulling this over and getting worried that we'd have to roll it back for compatibility with other tooling, but I found official documentation that this is the correct behavior: https://github.com/protocolbuffers/protobuf/blob/d4ec70fdd3380e7113e9e3c23cef2bea50dbf2e7/src/google/protobuf/timestamp.proto#L99-L124

[chrusty]

@xylo04 let's see how it goes. None of the unit tests were harmed. If we get any issues then I can put this behind another CLI parameter.