Make Service async fn-compatible

[sfackler]

Before this PR

The Service trait required that the call method's future be nameable and not borrow anything from self. This required lots of Arc cloning and manual future implementations.

After this PR

==COMMIT_MSG==
The Service trait now supports async fn call.
==COMMIT_MSG==

Using the new support added in Rust 1.75 (https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html), we can change Service's signature to allow the call method to be written as a normal async fn.

This significantly simplifies the internal service layers. Manual future implementations are now reserved for low level operations that can't be expressed with async/await.

Possible downsides?

Due to limitations of the initial implementation in the compiler, we currently require that the future returned by call is Send. In practice this isn't really a limitation since the future needs to be Send to work in a multithreaded runtime.

[changelog-app]

Generate changelog in changelog/@unreleased

What do the change types mean?

• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

Note: only one type should be chosen.

How are new versions calculated?

• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

The Service trait now supports async fn call.

Check the box to generate changelog(s)

• Generate changelog entry

[TheKeveloper]

Why can't we do

async fn call(&self), req: R) -> Result<T::Response, T::Error>

[sfackler]

We could, but that would add an extra layer of indirection - it would end up making a wrapper future that looked something like

enum WrapperFuture<F> {
    Init,
    Awaiting(F),
}

[TheKeveloper]

nit: I think we can avoid the mut on error

let error = if log_body { error.with_unsafe_param(....) } else { error }

[sfackler]

That's true, but IMO it's a bit more confusing.