Add witnessed_by_cid-equivalent helper to UcanBuilder API

[cdata]

As of #28 , we only recognize CIDs in the prf field of UCANs. However, the UcanBuilder API only accepts a Ucan as the value for the witnessed_by method that adds elements to the prf field. This is convenient because the builder automatically encodes the Ucan and adds its CID on your behalf. However, there are cases where you will have the CID handy but perhaps not the UCAN it points to. For this case, it can be convenient to add a witness by CID. We should enhance UcanBuilder to support this.

[cdata]

Maybe instead, it should be...

pub trait Authorization {
  async fn resolve_ucan<S: UcanJwtStore>(&self, &S) -> Result<Ucan>;
}

// Or...

pub enum Authorization {
  Ucan(Ucan),
  Cid(Cid),
  Jwt(String),
  UcanIpld(UcanIpld)
}

And then the builder signature is changed to something like...

pub struct UcanBuilder {
  // ...
 
  pub fn witnessed_by(mut self, authorization: &Authorization) -> Self { /* ... */ }

  // ...
}