feat: new System API ic0.subnet_self

[mraszyk]

This PR specifies a new System API ic0.subnet_self_size and ic0.subnet_self_copy that canisters can use to synchronously learn their own subnet ID. Requested on the forum.

[github-actions]

🤖 Here's your preview: https://vlbny-pqaaa-aaaam-abe3a-cai.icp0.io

[Dfinity-Bjoern]

Thank you Martin – that looks good from the spec side, let's have @dsarlis chime in for the execution team.

[michael-weigelt]

Tangentially related: I assume the size part is necessary because principals are specified to have variable length.
However, why not change the spec to commit to a fixed length for subnet_ids and simplify this interface? Same for canister_ids?

[dsarlis]

Looks good from the execution team's pov.

[mraszyk]

Tangentially related: I assume the size part is necessary because principals are specified to have variable length. However, why not change the spec to commit to a fixed length for subnet_ids and simplify this interface? Same for canister_ids?

For backwards-compatibility, this is practically impossible for existing APIs and thus we'd introduce an inconsistency with those by following the above suggestion.