[FEATURE] Allow connectors to override CloudEvent attributes via user properties (don't block or ignore them)

### Product

Rust SDK

### Azure Iot Operations version

azure_iot_operations_mqtt = "1.0.2"
azure_iot_operations_protocol = "1.0.0"
azure_iot_operations_services = "1.2.0"
azure_iot_operations_connector = "2.0.1"

### What happened?

When a connector forwards data through the managed ADR `DataOperationClient` /
`Forwarder`, the SDK generates the CloudEvent attributes (`id`, `source`, `type`,
`subject`, `specversion`, `time`, `datacontenttype`, `dataschema`) itself. A
connector currently has **no way to override** any of these: if it supplies a
user property with a reserved CloudEvent name, the message is **rejected at build
time** rather than the connector's value being used.

Request: a connector should be able to set/override any CloudEvent attribute by
passing it as a user property, and the SDK should honor that value instead of
blocking it or silently discarding it.

### List the steps to reproduce

_No response_

### Code of Conduct

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