Allow custom CompletionItemKind outside of LSP definition

[milanglacier]

Feature Description

I want to ask if we can allow customized CompletionItemKind when create backend for blink, aka something like this:

function M:get_completions(context, callback)
    callback {
        items = {
            {
                label = 'hello',
                kind = 'AI',
            },
            {
                label = 'hello world',
                kind = 'AI',
            },
        },
    }
end

Customized item kind outside of the lsp protocol definition are widely used in nvim-cmp, for example, copilot.cmp, codeium.nvim, minuet-ai.nvim are all using customized kind names.