Consider a must_use attribute on SagaResult

[smklein]

SagaResult describes the result of executing a saga, akin to Rust's native Result type:

steno/src/saga_exec.rs

Lines 1407 to 1415 in 8da3eda

	/**
	* Summarizes the final state of a saga execution
	*/
	#[derive(Clone, Debug)]
	pub struct SagaResult {
	pub saga_id: SagaId,
	pub saga_log: SagaLog,
	pub kind: Result<SagaResultOk, SagaResultErr>,
	}

This type is propagated back to the SecClient, where it may be acquired when awaiting the future returned from saga_create.

As this is the only mechanism for actually checking the success or failure of a saga, we should consider annotating it with the must_use attribute.

Note that this would align with the stdlib Result type attribute, which uses must_use to encourage callers to handle errors explicitly.