[subsystem-benchmarks] Save results to json (#3829)

Here we add the ability to save subsystem benchmark results in JSON
format to display them as graphs

To draw graphs, CI team will use
[github-action-benchmark](https://github.com/benchmark-action/github-action-benchmark).
Since we are using custom benchmarks, we need to prepare [a specific
data
type](https://github.com/benchmark-action/github-action-benchmark?tab=readme-ov-file#examples):
```
[
    {
        "name": "CPU Load",
        "unit": "Percent",
        "value": 50
    }
]
```

Then we'll get graphs like this: 

![example](https://raw.githubusercontent.com/rhysd/ss/master/github-action-benchmark/main.png)

[A live page with
graphs](https://benchmark-action.github.io/github-action-benchmark/dev/bench/)

---------

Co-authored-by: ordian <write@reusable.software>

polkadot/node/subsystem-bench/Cargo.toml

@@ -71,6 +71,7 @@ prometheus_endpoint = { package = "substrate-prometheus-endpoint", path = "../..
 prometheus = { version = "0.13.0", default-features = false }
 serde = { workspace = true, default-features = true }
 serde_yaml = { workspace = true }
+serde_json = { workspace = true }
 
 polkadot-node-core-approval-voting = { path = "../core/approval-voting" }
 polkadot-approval-distribution = { path = "../network/approval-distribution" }