fix(conf) infer vault references

### Summary

Kong escapes e.g. `"#"` with `"\#"`, and these need to be removed before
passing them to vault functions as otherwise reference like:

```
{vault://env/pg-password#1}
```

Doesn't work as it gets past as:

```
{vault://env/pg-password\#1}
```

This fixes that.

kong/conf_loader/init.lua

@@ -1539,13 +1539,14 @@ local function load(path, custom_conf, opts)
     local vault_conf = { loaded_vaults = loaded_vaults }
     for k, v in pairs(conf) do
       if sub(k, 1, 6) == "vault_" then
-        vault_conf[k] = v
+        vault_conf[k] = infer_value(v, "string", opts)
       end
     end
 
     local vault = require("kong.pdk.vault").new({ configuration = vault_conf })
 
     for k, v in pairs(conf) do
+      v = infer_value(v, "string", opts)
       if vault.is_reference(v) then
         if refs then
           refs[k] = v

spec/01-unit/03-conf_loader_spec.lua

@@ -1742,11 +1742,11 @@ describe("Configuration loader", function()
       helpers.setenv("PG_PORT", "5000")
 
       local conf = assert(conf_loader(nil, {
-        pg_port = "{vault://env/pg-port}"
+        pg_port = "{vault://env/pg-port#0}"
       }))
 
       assert.equal(5000, conf.pg_port)
-      assert.equal("{vault://env/pg-port}", conf["$refs"].pg_port)
+      assert.equal("{vault://env/pg-port#0}", conf["$refs"].pg_port)
     end)
   end)
 end)