feat(spans): Infer span.op if not defined

[jjbayer]

Add global option that allows inferring the span op, with rules like this:

{
    "span_op_defaults": {
       "rules": [{
            "condition": {
                "op": "not",
                "inner": {
                    "op": "eq",
                    "name": "span.data.messaging\\.system",
                    "value": null,
                },
            },
            "value": "message"
        }]
    }
}

ref: #3637