refactor(pdk) vault pdk to be more like rest of the pdk modules

### Summary

Refactor Vault PDK to follow other Kong PDK modules. This means that functions
are created inside `.new` function. This has benefit of being able to access
up-value `self`, which means that no direct references to global `kong` is needed.

In general, it makes testing and mocking easier too.

I need this so I can pass some initial configuration very early on when Kong does
process secrets resolving of Kong configuration references.

kong/cmd/vault.lua

@@ -42,7 +42,6 @@ end
 
 
 local function get(args)
-  local vault = require "kong.pdk.vault".new()
   if args.command == "get" then
     local reference = args[1]
     if not reference then
@@ -51,6 +50,8 @@ local function get(args)
 
     init_db(args)
 
+    local vault = kong.vault
+
     if not vault.is_reference(reference) then
       -- assuming short form: <name>/<resource>[/<key>]
       reference = fmt("{vault://%s}", reference)

kong/db/schema/init.lua

@@ -2,11 +2,9 @@ local tablex       = require "pl.tablex"
 local pretty       = require "pl.pretty"
 local utils        = require "kong.tools.utils"
 local cjson        = require "cjson"
-local vault        = require "kong.pdk.vault".new()
+local is_reference = require "kong.pdk.vault".new().is_reference
 
 
-local is_reference = vault.is_reference
-local dereference  = vault.get
 local setmetatable = setmetatable
 local getmetatable = getmetatable
 local re_match     = ngx.re.match
@@ -1744,7 +1742,7 @@ function Schema:process_auto_fields(data, context, nulls, opts)
               refs = { [key] = value }
             end
 
-            local deref, err = dereference(value)
+            local deref, err = kong.vault.get(value)
             if deref then
               value = deref
 
@@ -1768,7 +1766,7 @@ function Schema:process_auto_fields(data, context, nulls, opts)
               for i = 1, count do
                 if is_reference(value[i]) then
                   refs[key][i] = value[i]
-                  local deref, err = dereference(value[i])
+                  local deref, err = kong.vault.get(value[i])
                   if deref then
                     value[i] = deref
                   else