add json schema

[retogerber]

add the json schema as well next to the yaml schema. This allows to use linters interactively which can make it easier to create the benchmark yaml. For instance for VS code:
install extension: https://marketplace.visualstudio.com/items?itemName=redhat.vscode-yaml
in VS code add add to settings.json:

    "yaml.schemas": {
        "https://raw.githubusercontent.com/omnibenchmark/omni-schema/refs/heads/add_json_schema/src/omni_schema/schema/omni_schema.json": [
            "**.yaml"
        ]
    }

We could also think about integrating this to https://www.schemastore.org/json/ this would make it pretty universally usable.
What do you think @imallona @DanInci

[imallona]

Sorry missed this. I don't see an immediate use but I see no harm neither, looks good to me.

[DanInci]

@retogerber can be closed, since we don't use omni-schema anymore