Deprecate `ReducerContext::identity` in favor of `ReducerContext::database_identity`

[gefjon]

We have multiple reports of users (e.g. in this thread in the public Discord) being confused that ctx.identity() in module code reads the database identity, and calling this method when they intend to use ctx.sender. We should rename ctx.identity() to be more clear; ctx.database_identity() seems obvious and unambiguous. The path to doing this without a breaking change is to add the new method and to mark the old one as #[deprecated].

Also make the corresponding change in C# modules, and figure out what their equivalent of #[deprecated] is.

After doing this, add a new ticket to review the client's conn.identity() and consider whether it should be renamed to conn.client_identity().

[Korenn]

"module" could work as well, or "module_identity"

[Obsolete] is how you mark things as deprecated in C#
https://stackoverflow.com/questions/1759352/how-to-mark-a-method-as-obsolete-or-deprecated

[JulienLavocat]

This would be great, even when helping users debug this specific issue I'm never 100% sure which one (identity or sender) is correct

[cloutiertyler]

@Korenn "module" is not the correct terminology, as the module refers specifically to the webassembly code which does not have an identity. One could imagine having the same module for multiple databases (each with their own identity).