Provide casting option for attributes of created telemetry #24
Description
With #16, the dash0 CLI can create telemetry. All attributes are specified as key-value pairs (<key>=<value>) and result in attributes with string values. We should add support a syntax to specify the type of the value to cast to, e.g., boolean(foo=true) resulting in the attribute foo having the boolean true as value. The proposed syntax, with typing around the entire key-value pair, may need tweaking, but it does prevent (unlikely) situations where actual values are misunderstood by a mix of value and typing information.