Fix client operation name collisions with the standard library prelude (#2696)

## Motivation and Context
Operations named `Send` or `Sync` (and probably others) were colliding
with the types in the standard library prelude and causing compiler
errors. This PR adds tests that include all the type names from the Rust
prelude, and fixes the compiler errors they cause.

In the future, the `no_implicit_prelude` attribute can be added to
certain code generated modules to better enforce that there can't be
name collisions, but for now, the `tokio::test` macro doesn't compile
with that attribute enabled (and likely other macros from other
libraries).

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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