`CatalystSignedDocument` signing logic

[Mr-Leshiy]

Summary

Implement a signing logic for the CatalystSignedDocument object.

Description

• Inside Content::new method add content: Vec<u8> validation based on the ContentType.
  something like this

match content_type {
ContentType::Json =>  serde_json::from_slice::<serde_json::Value>(&content)?,
ContentType::Cbor => minicbor::decode::<minicbor::data::Token>(&content)?,
}

• Add a new method verify for CatalystSignedDocument struct. which will verify the underlying signatures and all underlying Uuid types (do the conform to the v7 or v4 versions).

pub fn verify(&self, pk_getter: &impl Fn(&Kid) -> VerifyingKey) -> Result<(), ProblemReport>

• Add a new method sign for CatalystSignedDocument struct. Which will add a new signature.

pub fn sign(&self, sk: SecretKey, kid: Kid);

• some unit tests