fix: Require tokio/rt feature for sync scenarios

[flosse]

This should fix #191 but I'd feel better if there is a test/configuration where the CI detects such bugs.
@uklotzde do you have an idea how to provoke the issue?

[uklotzde]

Just fixing the issue without a regression test that triggers the dependency/feature error is brittle.

[uklotzde]

cargo check --no-default-features --features rtu-sync fails.

Ideally, all or at least selected feature combinations should be checked by the CI builds. Using all possible combinations does not make much sense, but checking single features in isolation like demonstrated here would help.

[uklotzde]

Another argument to avoid overusing features for crate extensions. Using separate sync and server crates would be more appropriate.

The sync stuff doesn't fit in here at all. Offering a sync-over-async bridge layer is pointless for an async-focused crate IMHO.

[flosse]

The sync stuff doesn't fit in here at all. Offering a sync-over-async bridge layer is pointless for an async-focused crate IMHO.

I agree!

[uklotzde]

After force pushing GitHub noticed the new workflow action.