It's not clear from `metaModel.json` that `MessageActionItem` can contain arbitrary additional fields

The generator in https://github.com/microsoft/lsprotocol uses metaModel.json as the source of truth about the LSP protocol.

While the docs for ClientShowMessageActionItemOptions.additionalPropertiesSupport indicate that MessageActionItem objects can contain arbitrary additional attributes beyond what is shown in the spec, the description of MessageActionItem itself doesn't indicate this:

		{
			"name": "MessageActionItem",
			"properties": [
				{
					"name": "title",
					"type": {
						"kind": "base",
						"name": "string"
					},
					"documentation": "A short title like 'Retry', 'Open Log' etc."
				}
			]
		},

I think it would be more standard to have an optional data field of type LSPAny, like on the Diagnostic object.

Is there any way that we can make this aspect of MessageActionItem clear in the json spec so tools that consume metaModel.json can support it correctly?

Related:

• https://github.com/microsoft/lsprotocol/issues/411
• https://github.com/openlawlibrary/pygls/issues/518

Start with the MessageActionItem entry in metaModel.json and compare it with the Diagnostic object and the linked LSP specification section. Review related issues 411 and pygls issue 518; done means the JSON model clearly communicates the support for arbitrary additional fields to consuming tools.