extend api with optional parameter `document_url`

[bernhardreiter]

Both ISuBA and https://github.com/gocsaf/forwardertarget have implemented an extended API, that should be backwards compatible if the optional form parameter document_url is ignored.

See https://github.com/ISDuBA/ISDuBA/blob/v0.13.0/docs/forwarder.md#forward-request for the extensions:

• naming the three values for the validation_status
• allowing optional document_url, it is optional because not all forwarding applications offer a URL that can be called externally. E.g. csaf_downloader does not.