fix(aws-lambda): lazily initialize aws library (#11614)

### Summary

Lazily initializes AWS library on a first use, to remove startup delay caused
by AWS metadata discovery.

Signed-off-by: Aapo Talvensaari <aapo.talvensaari@gmail.com>

kong/db/dao/plugins.lua

@@ -355,29 +355,5 @@ function Plugins:get_handlers()
   return list
 end
 
-function Plugins:execute_plugin_init()
-  local handlers, err = self:get_handlers()
-  if not handlers then
-    return nil, err
-  end
-
-  local errors
-
-  for _, handler in ipairs(handlers) do
-    if implements(handler.handler, "init") then
-      local ok, err = pcall(handler.handler.init, handler.handler)
-      if not ok then
-        errors = errors or {}
-        errors[#errors + 1] = "on plugin '" .. handler.name .. "': " .. tostring(err)
-      end
-    end
-  end
-
-  if errors then
-    return nil, "error executing plugin init: " .. table.concat(errors, "; ")
-  end
-
-  return true
-end
 
 return Plugins

kong/init.lua

@@ -629,8 +629,6 @@ function Kong.init()
   -- Load plugins as late as possible so that everything is set up
   assert(db.plugins:load_plugin_schemas(config.loaded_plugins))
 
-  assert(db.plugins:execute_plugin_init())
-
   if is_stream_module then
     stream_api.load_handlers()
   end

kong/plugins/aws-lambda/handler.lua

@@ -8,6 +8,7 @@ local ngx_update_time = ngx.update_time
 local kong = kong
 local meta = require "kong.meta"
 local constants = require "kong.constants"
+local aws_config = require "resty.aws.config" -- reads environment variables, thus specified here
 local VIA_HEADER = constants.HEADERS.VIA
 local VIA_HEADER_VALUE = meta._NAME .. "/" .. meta._VERSION
 
@@ -30,18 +31,24 @@ local function get_now()
 end
 
 
+local function initialize()
+  AWS_GLOBAL_CONFIG = aws_config.global
+  AWS = aws()
+  initialize = nil
+end
+
+
 local AWSLambdaHandler = {
   PRIORITY = 750,
   VERSION = meta.version
 }
 
-function AWSLambdaHandler:init()
-  AWS_GLOBAL_CONFIG = require("resty.aws.config").global
-  AWS = aws()
-end
-
 
 function AWSLambdaHandler:access(conf)
+  if initialize then
+    initialize()
+  end
+
   -- The region in plugin configuraion has higher priority
   -- than the one in environment variable
   local region = conf.aws_region or AWS_REGION