feat: add callbacks

add events upon request start/stop so that users can customize rest.nvim
without modifying the plugin too much.

lua/rest-nvim/init.lua

@@ -95,7 +95,20 @@ rest.run_request = function(req, opts)
     request.highlight(result.bufnr, result.start_line, result.end_line)
   end
 
+  local request_id = vim.loop.now()
+  local data = {
+        requestId = request_id,
+        request = req
+      }
+
+  vim.api.nvim_exec_autocmds("User", {
+      pattern = "RestStartRequest",
+      modeline = false,
+      data = data
+    })
   local success_req, req_err = pcall(curl.curl_cmd, Opts)
+  vim.api.nvim_exec_autocmds("User", { pattern = "RestStopRequest", modeline = false,
+      data = vim.tbl_extend("keep", { status = success_req, message = req_err }, data)  })
 
   if not success_req then
     vim.api.nvim_err_writeln(