Introduce additional span status for cancellations

[thompson-tomo]

What are you trying to achieve?

I want to have a way to set the status of a span as cancelled as opposed to error when the application decides to cancel a currently running operation.

As part of setting this status it should be permissible to set the status message/description to provide additional context such as was it a losing hedge or was it user initiated.

In terms of status order it should be between unset & error.

Additional context.

• Request Hedging semantic-conventions#1712
• Convention for cancelled spans semantic-conventions#560

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