feat(nns): Omit logos for CreateServiceNervousSystem proposals

[jasonz-dfinity]

Currently, when the proposal actions are returned in a list_proposals response, the logos can be omitted as they are large. In this PR we retain this behavior for self_describing_action.

In addition:

1. The pb->api conversion of SelfDescribingValue is refactored to take a reference instead of consuming the pb
2. The api->pb conversion of SelfDescribingValue is removed since it's not needed

[github-actions]

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No behavior change (behind feature flag)