RequestForInterest: Support decoding application/x-www-form-urlencoded

Support simple www-form-urlencoded message bodies for storage requests.

Message body format:
field=value&field2=value

-------

This is not intended to be pulled upstream as is, but rather a pull request to see interest in having it upstream.

I've added a application/x-www-form-urlencoded Decoder to allow using simple html form encoded message bodies directly against the backend storage.

This could be used via HTML e.g.:

```html
<form method=POST" url="storage/test">
  <input type="text" name="field" value="x" />
  <input type="text" name="field2" value="y" />
</form>
```

Or via command line tools, e.g.:

```console
curl --request POST http://example.com/app/storage/test --data "field=x&field2=y"
```

If this sounds interesting, I'll clean up/improve some code and do a new request.