nixlbench/gusli: add --gusli_try_use_uring configurable option (#1388)

Add --gusli_try_use_uring flag to enable the io_uring engine in the
GUSLI backend. Can be passed via CLI or --config_file TOML.

When set, gusli_backend will call io.params.set_try_use_uring(true) on
each IO request. README updated with the new parameter.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **New Features**
* Added gusli_try_use_uring option to attempt using io_uring for the
GUSLI backend (default: disabled; configurable via CLI or config file).

* **Documentation**
  * Usage and parameter lists updated to document the new option.

* **Behavior**
* The selected "try use uring" setting is logged during backend
initialization and propagated through Gusli I/O paths.

* **Build**
* Gusli build configuration updated to enable io_uring support by
default.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: Omer Brezel <omer.b1@dell.com>
Co-authored-by: Adit Ranadive <aranadive@nvidia.com>