benchmark: Add "unstable-testing" feature

We may sometimes need to benchmark / integration test some private stuff.
There is no good way to do this. The least bad way seems to be to
re-export such private APIs under an unstable feature, and mark tests or
benchmarks that require them as depending on this feature.

It's worth noting that using "required-features" cargo attribute won't
automatically enable the feature - it will just prevent the target from
existing unless the feature is enabled.
It means that after this commit `cargo bench` won't run this benchmark.
Instead you need to run "cargo bench --features unstable-testing" or
"cargo bench --all-features".

Author: Lorak-mmk

scylla/Cargo.toml

@@ -39,6 +39,7 @@ full-serialization = [
     "num-bigint-04",
     "bigdecimal-04",
 ]
+unstable-testing = []
 
 [dependencies]
 scylla-macros = { version = "0.7.0", path = "../scylla-macros" }
@@ -96,6 +97,7 @@ time = "0.3"
 [[bench]]
 name = "benchmark"
 harness = false
+required-features = ["unstable-testing"]
 
 [lints.rust]
 unnameable_types = "warn"