feat: initial cstoken support

[gusquadri]

Introduce the initial implementation of cstokens, which currently appears to be in a very early stage in WA Web, as the feature flag is usually disabled in my tests.

It should receive the server-side nctsalt to generate cstokens (only through history sync and app sync events), and should only be generated when the tctoken is missing and the salt exists (as a fallback).

Initial question for Tulir:

• Should we introduce an in-memory cache for these salts, or should we just store them in the database?

[purpshell]

• Should we introduce an in-memory cache for these salts, or should we just store them in the database?

I think these salts need to be stored inside the Store since it seems like WA doesn't regularly keep them updated per session. They can be loaded into memory during Connection to reduce DB overhead though.