chore(dkg): remove summary's transcripts for remote subnets (1/4) - #11220

Since the work performed to speedup the latency of remote DKG requests, their results are delivered as part of data blocks instead of summaries. The transcripts_for_remote_subnets field in summaries is thus now unused. This PR begins the migration to remove it entirely. Once it is, the batch delivery can be rewritten more cleanly as we would know that summary heights never contain any consensus responses.

We cannot simply replace the field with a BackwardsCompatible<Vec<_>, _> (i.e. replace the Vec<_> with an Option<Vec<_>>) because the field is a repeated, and an empty vector is not differentiable from an absent one. In contrast, Some(vec![]) and None have different hashes. For that reason, we need to add a new sentinel field to the protobuf definition that will be set to true to explicitely indicate that the field is absent. The entire rollout can be reviewed by looking at the stacked PRs and is described below (together with justifying why upgrades & downgrades are safe).

• V0 (current) <-> V1 (after this PR):
  • The goal of V1 is to keep the exact same representation (including hash) of V0's summaries while still understanding the sentinel and compute the same hash as V2 for V2's summaries.
  • Upgrade V0 -> V1: A V0's summary doesn't set the sentinel so the field deserializes as Some(vec![]) and the hash of an empty vector is appropriately computed.
  • Rollback V1 -> V0: a V1's summary doesn't set the sentinel just yet. V0 does not read it anyways and always includes an empty vector in the hash, same as V1.
• V1 <-> V2 (#11221):
  • The goal of V2 is to actually stop hashing an empty vector, and instead just ignore it.
  • Upgrade V1 -> V2: V2 still understands Some(vec![]) so will compute V1's summary's hash identically. For its own summaries, it starts to fill the sentinel (because the transcripts are None).
  • Rollback V2 -> V1: V1 understands the sentinel and thus ignores the transcripts while computing the hash.
• V2 <-> V3 (#11223):
  • The goal of V3 is to remove the field entirely.
  • Upgrade V2 -> V3: V3 doesn't know about the transcripts anymore (the field is removed). V2 created a summary with None transcripts meaning their hash was not included, good.
  • Rollback V3 -> V2: V3 unconditionally sets the sentinel to true, meaning V2 will interpret the transcripts as None and not try to hash them.
• V3 <-> V4 (#11224):
  • The goal of V4 is to remove the introduced sentinel field.
  • Upgrade V3 -> V4: V4 will ignore the sentinel field set by V3.
  • Rollback V4 -> V3: V3 will deserialize a summary with an unset sentinel, but it does not read it anyways.