Werk #14676: Item description in CheckParameterRuleSpecWithItem mandatory

This change might break existing MKPs.

When writing a new rulespec for a check with an item the item_spec could be left blank. However those are needed to properly render the rulespec and allow to enable a rulespec only for specific services of a check plugin.

Now the item_spec parameter is mandatory. The item_name, and item_help parameters have been removed, with werk #14677. Below is an example how to register a new rulespec for a check with an item.

    rulespec_registry.register(
        CheckParameterRulespecWithItem(
            check_group_name="a_check",
            item_spec=lambda: TextInput(title="fitting item name", help="inline help text"),
            group=RulespecGroupCheckParametersApplications,
            parameter_valuespec=_parameter_valuespec_network,
            title=lambda: "A good title",
        )
    )

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