Add UNSPECIFIED zero value for ActorSnapshotTagScope

[Luiz Oliveira (laoj2)]

Fixes #897

• Tests pass
• Appropriate changes to documentation are included in the PR

[Eitan Yarmush (EItanya)]

I don't fully understand why this is necessary? If the default is ATESPACE, why can't that be 0?

[Luiz Oliveira (laoj2)]

I don't fully understand why this is necessary? If the default is ATESPACE, why can't that be 0?

substrate's API style guide (https://github.com/agent-substrate/substrate/blob/main/docs/api-style-guide.md#51-enum-naming) made an explicit decision of requiring _UNSPECIFIED values for the "not set" case so we can avoid issues like this:

1. We add a new ACTOR_SNAPSHOT_TAG_SCOPE_XXX to the enum
2. A client gets released before the server.
3. The stale server receives a request with the new ACTOR_SNAPSHOT_TAG_SCOPE_XXX
4. The proto field accessor in the server returns the default value: the server silently gets _ATESPACE, potentially making a wrong decision instead of failing