Representation of Singletons in StateProofs and BlockStream state changes

As described in this [notion](https://www.notion.so/swirldslabs/Singletons-in-StateProofs-and-BS-State-Changes-1fa7c9ab259180379b19eee93705553d?pvs=4), we currently have `SingletonIdentifier` that is different for each Singleton state.

We could replace all the singletons in `StateIdentifier` with one and then add a singleton enum and add the type of singleton into the value of `SingletonUpdateChange`

Start with the linked Notion design and inspect the usages of SingletonIdentifier, StateIdentifier, and SingletonUpdateChange in the repository. Trace how StateProofs and BlockStream state changes represent singletons, then verify that the implementation uses one singleton identifier, an enum for singleton kinds, and includes that kind in each SingletonUpdateChange value.

Written by the indexing model from the issue text.