feat: custom fn_transform_cmd (closes #1927)

Enables custom command building, for example git grep with glob:
```lua
require("fzf-lua").live_grep({
  cmd = "git grep -i --line-number --column --color=always",
  fn_transform_cmd = function(query, cmd, opts)
    -- Uncomment to debug
    -- io.stdout:write(string.format("[DEBUG] %s\n", cmd .. query))
    return cmd .. query
  end
})
```

Author: ibhagwan

lua/fzf-lua/make_entry.lua

@@ -219,6 +219,14 @@ M.preprocess = function(opts)
     end
   end
 
+  -- For custom command transformations (#1927)
+  opts.fn_transform_cmd =
+      load_config_section("__resume_data.opts.fn_transform_cmd", "function", true)
+  if opts.fn_transform_cmd then
+    local query = argv(nil, opts.debug)
+    opts.cmd = opts.fn_transform_cmd(query, opts.cmd:gsub("{argv.*}", "") ,opts)
+  end
+
   -- nifty hack to avoid having to double escape quotations
   -- see my comment inside 'live_grep' initial_command code
   if opts.argv_expr then