Determine payload value type based on enum

[pohlm01]

For the payloads in the intervals, the data type of the value is determined by the type of the payload.
The VTN should make sure that the data type it expects for the provided type matches the one it received.

openleadr-rs/openleadr-wire/src/event.rs

Lines 237 to 246 in a0d0128

	/// Represents one or more values associated with a type. E.g. a type of PRICE contains a single float value.
	#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
	pub struct EventValuesMap {
	/// Enumerated or private string signifying the nature of values. E.G. \"PRICE\" indicates value is to be interpreted as a currency.
	#[serde(rename = "type")]
	pub value_type: EventType,
	/// A list of data points. Most often a singular value such as a price.
	// TODO: The type of Value is actually defined by value_type
	pub values: Vec<Value>,
	}

[pohlm01]

@benjaminedwardwebb did a great job in #124 by validating if the type of the values matches what is expected from the value_type. Still, we keep this issue open as we strive for a cleaner version in the future: Instead of validating if the type is correct, we first want to parse it properly using a custom serde deserialize implementation. See the comments in #124 for the full insight.