feat(sns): Automatically advance SNS target version upon opt-in

[aterga]

This PR implements a simple extension of the AdvanceSnsTargetVersion mechanism, allowing SNSs to opt-in for getting target versions advanced to the latest available SNS version blessed by the NNS community.

[anchpop]

One possibility would be to have a staged rollout by default (e.g. only a 1% chance per refresh of setting the target version). This would mean that we'd have some time to stop the release if we start noticing problems. I think staged rollouts are a common practice for this reason.

[aterga]

One possibility would be to have a staged rollout by default (e.g. only a 1% chance per refresh of setting the target version). This would mean that we'd have some time to stop the release if we start noticing problems. I think staged rollouts are a common practice for this reason.

I think that would complicate the feature too much in this case. In the unlikely event of a critical bug that goes through NNS blessing, an SNS that opted in for this feature would need to pass another proposal to opt out of it, and then it could regain manual control again.

[github-actions]

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