Fix compile errors of stand-alone features

[haerdib]

Fixes the compile errors of standalone features. The following was changed:

• Added additional CI checks
• Added futures-util to default import because the async code needs it to compile. Because the sync code does not, I only added the minimum needed package (which is -util and not the full futures).
• tungstenite-client and ws-client now include the sync-api feature, because they are only work in sync mode anyway.
• fixed async no-std compiliation by adding #[cfg(not(feature = "sync-api"))] use alloc::boxed::Box; : async-trait automatically changes async return values into boxed values.

closes #644

[Niederb]

I don't quite understand why the changes with Box are needed. I assume you had compilation errors. Can you tell me how I can reproduce them?

#[cfg(not(feature = "sync-api"))]
use alloc::boxed::Box;

[haerdib]

See the PR description:

• Fixed async no-std compiliation by adding #[cfg(not(feature = "sync-api"))] use alloc::boxed::Box; : async-trait automatically changes async return values into boxed values.

For reproducing: Simply remove the use alloc::boxed::Box; and run cargo check --no-default-features

[Niederb]

Now I understood. Thanks