Rust bindings callbacks aren't Send + Sync

[maddeleine]

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Problem:

None of our callbacks are of type Send + Sync. This is a problem because config implements Send + Sync, which means that it can be mutated and sent across threads safely. And configs own those callbacks. So basically the callbacks are not thread-safe when they should be.

Solution:

We probably need to change all the callbacks to be Send + Sync. I think this is a breaking change. Essentially what this means is that people who pull in this change will have to rewrite their callbacks to be thread-safe, which is good, but it will be annoying for them.

Requirements / Acceptance Criteria:

Might want to look into if they need to be 'static as well. Also is there a programmatic way of checking that anything on the config is send + sync?