Rust client SDK: rework EventContext into multiple types

[gefjon]

Description of Changes

A repeated pain point with the post-0.12 client SDK API
has been excessive unnecessary pattern-matching or unwrapping
on the ctx.event within reducer callbacks.
Previously, these had ctx.event: Event,
with the type system not knowing that said Event would always be Event::Reducer.
This was in a misguided attempt to reduce proliferation of context types.

As of this PR, we have different event context types for each category of callback,
which allows us to inform the type system precisely of the type of event.

The event context types:

• EventContext, passed to row callbacks.
  • Exactly the previous type.
  • In the future we may examine enum Event more closely and see if any of its variants are unreachable in row callbacks. If they are, we can deprecate those variants.
• ReducerEventContext, passed to reducer callbacks.
  • event: ReducerEvent<Reducer>.
• SubscriptionEventContext, passed to subscription applied and unapplied callbacks.
  • No event field needed, or event: ().
• ErrorContext, passed to various on-error callbacks.
  • event: Error.

Changes to callbacks:

• DbConnectionBuilder::on_disconnect now takes FnOnce(&ErrorContext).
  • For "normal" disconnects, this will have event: Error::Disconnected.
  • Is this better than having a separate DisconnectContext, which would likely also hold event: Error?
• DbConnectionBuilder::on_connect_error now takes FnOnce(&ErrorContext).
• on_{reducer} now takes FnMut(&ReducerEventContext, ...Args).
• SubscriptionBuilder::on_applied and SubscriptionHandle::unsubscribe_then now take FnOnce(&SubscriptionEventContext).
• SubscriptionBuilder::on_error now takes FnOnce(&ErrorContext).

Notes for reviewers

Based on #2169 , which in turn is based on #2111 . Use the "show changes from all commits" dropdown in the upper-left of the review window to isolate this PR's changes, which start from 8234155a.

Most of this diff, linewise, is in autogenerated code, which you can largely or completely ignore.

Question

I have kept all the event context types having a field ctx.event (except SubscriptionEventContext, which lacks any useful information to put there), of varying type. We could, alternatively, give this field a per-context name, like ctx.error for ErrorContext, ctx.reducer_event for ReducerEventContext, and... well, actually those are the only two that would change.

API and ABI breaking changes

Ayep, breaks the Rust client SDK API pretty comprehensibly. See crates/sdk/examples/quickstart-chat/main.rs for a sense of the changes.

Expected complexity level and risk

1: The changes are essentially mechanical and do not change any semantics. Our codegen is extensively tested.

Testing

• I believe automated testing to be sufficient.

[kazimuth]

This is going to want to be rebased onto #2184 as well probably