refactor(cmp): add optional where necessary (#4557)

## Description
Makes it easier to disable it for users who want to try other completion
engines, without it being pulled back by some Extra.

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## Related Issue(s)
None
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## Checklist

- [x] I've read the
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guidelines.

lua/lazyvim/plugins/extras/lang/rust.lua

@@ -9,6 +9,7 @@ return {
   -- Extend auto completion
   {
     "hrsh7th/nvim-cmp",
+    optional = true,
     dependencies = {
       {
         "Saecki/crates.nvim",