Support async-generic feature flags in cgp-async

[soareschen]

• Introduce the following feature flags to cgp-async:
  • async
  • send
  • sync
  • static
  • full - default feature with all enabled
• Introduce the following traits in cgp-async:
  • MaybeSend - alias to Send when the send feature is enabled, otherwise nothing.
  • MaybeSync - alias to Sync when the sync feature is enabled, otherwise nothing.
  • MaybeStatic - alias to 'static when the static feature is enabled, otherwise nothing.
• Update the Async trait from Sized + Send + Sync + 'static to MaybeSend + MaybeSync + MaybeStatic.
  • The Sized constraint is removed from Async to allow use inside dyn traits.
• Update the #[async_trait] macro to desugar async functions to return impl Future<Output: MaybeSend>.
  • Use of #[async_trait] now requires import of cgp::prelude::* to allow MaybeSend to be auto imported.
• cgp-async now re-exports cgp_sync::strip_async if the async feature is not enabled.
  • i.e. async functions are desugared into sync functions if the async feature is disabled.
• Crates such as cgp and cgp-core offers the full feature, which can be disabled to disable the indirect default features in cgp-async.