feat(sns): Add new neuron command SetFollowing

[aterga]

This PR adds a new neuron management command, SetFollowing. This command is not currently implemented and cannot be used; we just introduce the required API level changes.

[github-actions]

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Done

[aterga]

The PR breaks the API by moving from followees : vec NeuronId; to followees : vec Followee;.

• Why I want this: This is to accommodate for more (optional) fields, e.g., the neuron's alias. The requirement wasn't clear to me a few days ago, but now it's clear that the frontends would benefit from this API.
• This is safe because we're breaking the API that cannot be used for now (so the internal types are not used and the frontends were not yet adjusted to use the external types).