[stacked] Stop using #[async_trait] in conjure-macros

[sfackler]

Before this PR

Async custom clients and endpoints traits had to use the #[async_trait] macro.

After this PR

==COMMIT_MSG==
Async custom clients and endpoints no longer use the #[async_trait] macro.
==COMMIT_MSG==

Possible downsides?

Due to the lack of support for applying bounds to the anonymous future return types, the macro currently rewrites the trait definitions to add Send bounds to the trait definition. Still an improvement over #[async_trait] though.

[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

Async custom clients and endpoints no longer use the #[async_trait] macro.

Check the box to generate changelog(s)

• Generate changelog entry

Invalidated by push of eedea7e