Refactor `C8yCreateEvent` struct

[rina23q]

Is your refactoring request related to a problem? Please describe.
This request is originated by #2421 (comment)

the need to create a different representations with externalSource for JSON over MQTT and source for HTTP. We should only be needing just one struct where that source field can be an enum of either externalSource or source. If this isn't trivial, you may address that refactoring in a different PR.

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C8yCreateEvent is used for both cases, creating an event via JSON over MQTT and HTTP.
For HTTP, source holds the internal ID of a device, and for MQTT, extras holds the external ID of a device as externalSource.

The current implementation is not clear what the caller of the API needs to provide to the struct.

thin-edge.io/crates/core/c8y_api/src/json_c8y.rs

Lines 26 to 40 in 9b1ce24

	pub struct C8yCreateEvent {
	#[serde(skip_serializing_if = "Option::is_none")]
	pub source: Option<C8yManagedObject>,
	#[serde(rename = "type")]
	pub event_type: String,
	#[serde(with = "time::serde::rfc3339")]
	pub time: OffsetDateTime,
	pub text: String,
	#[serde(flatten)]
	pub extras: HashMap<String, Value>,
	}

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