Rust client SDK: send SubscribeMulti and UnsubscribeMulti

[gefjon]

Description of Changes

Based on #2278 .

This commit modifies the Rust client SDK to use the new setwise subscription API.

This includes:

• Changing SubscriptionBuilder::subscribe to accept any implementor of IntoQueries, including single strings and also arrays of strings, for various definitions of "string" and "array." And fiddling a bit with the implementations of IntoQueries.
• Sending the SubscribeMulti and UnsubscribeMulti websocket messages instead of Subscribe and Unsubscribe.
• Stubbing the handling of SubscribeApplied and UnsubscribeApplied websocket messages, since those should now be unreachable.
• Handling the SubscribeMultiApplied and UnsubscribeMultiApplied messages instead.
• Simplifying the quickstart-chat and test-client clients to just subscribe to one big set, where previously they had a kind of wonky semaphore-based scheme to wait for all queries to be applied.

New syntax

You can now do:

ctx.subscription_builder()
  .subscribe(["SELECT * FROM user", "SELECT * FROM client"]);

The array passed here may be a [T; N] (fixed-size array), a &[T] (slice), Vec<T> (vec) for a few different string-like Ts, or a Box<[Box<str>]>.

You can also still subscribe to a single query without wrapping it in an array, like:

ctx.subscription_builder()
  .subscribe("SELECT * FROM user");

The string here may be &str, String or Box<str>. These are the same "string-like" types which can be put in arrays as above.

API and ABI breaking changes

I have not been super careful about the changed implementations of IntoQueries, so minor incompatibilities are possible between our most recent 1.0-rc release and this patch in terms of valid types to pass to SubscriptionBuilder::subscribe. Common cases, like &str and String, still work.

Expected complexity level and risk

1

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• I believe automated testing is sufficient.

[gefjon]

Attempting to rebase has proved once more to have been a mistake. Merging seems to have worked fine, though. Sorry for the confusion and force-pushes!