Support tags in RPC server tasks

[drewbanin]

Describe the feature

The rpc server should accept an arbitrary set of key/value pairs for non-builtin task methods. These tags should be semantically meaningless, but should also be returned in the output of the ps and poll methods.

Example rpc call:

{
	"jsonrpc": "2.0",
	"method": "run",
	"id": "123",
	"params": {},
	"tags": {
	    "branch": "fix/orders",
	    "commit": "#abc123"
	}
}

Resulting ps output:

{
    "result": {
        "rows": [
            {
                "task_id": "7226e841-a870-4a52-96da-32cad6f78f2b",
                "request_id": "123",
                "request_source": "127.0.0.1",
                "method": "run",
                "state": "success",
                "start": "2019-10-10T13:41:56.389109Z",
                "end": null,
                "elapsed": 0.91682,
                "timeout": null,
                    "tags": {
                        "branch": "fix/orders",
                        "commit": "#abc123"
                    }
            }
        ]
    },
    "id": "123",
    "jsonrpc": "2.0"
}

Resulting poll output:

{
    "result": {
        "results": [...],
        "generated_at": "2019-10-10T13:41:57.282238Z",
        "elapsed_time": 0.8061051368713379,
        "logs": [...],
        "status": "success"
        "tags": {
            "branch": "fix/orders",
            "commit": "#abc123"
        }
    },
    "id": "123",
    "jsonrpc": "2.0"
}

[beckjake]

Putting this data at the top level would violate the json-rpc spec be annoying to implement on the server side. Could we put it into params, instead?