Add support for `x-www-form-urlencoded`

[shrsv]

The following request cannot be made presently using Lama2:

curl --request POST \
  --url http://localhost:3000/oauth/token \
  --header 'Authorization: Basic YXBwbGljYXRpb246c2VjcmV0' \
  --header 'Content-Type: application/x-www-form-urlencoded' \
  --data grant_type=password \
  --data username=pedroetb \
  --data password=password

We have the MULTIPART keyword, but that is not exactly x-www-form-urlencoded. The difference between these two options can be found at SO

Maybe we could have something as simple as:

POST
FORM
http://xyz

a=b
c=d