chore(attestations): add STRING materials to referrer graph

[jiparis]

With this change, STRING materials are added to referral graph.

This is an example of chainloop att add --value "hello string" sent in two different attestations:

➜ cldev discover --digest sha256:cab0b7342c65f6aba6a3c328e91e7c70bb26314b8905c20b68ef503463dd0e52
{
   "digest": "sha256:cab0b7342c65f6aba6a3c328e91e7c70bb26314b8905c20b68ef503463dd0e52",
   "kind": "STRING",
   "downloadable": false,
   "public": false,
   "createdAt": "2024-12-18T23:08:57.435661Z",
   "references": [
      {
         "digest": "sha256:392c8d0d5402225250362919888eae781df9ee5e32870a90e215086f52dbf801",
         "kind": "ATTESTATION",
         "downloadable": true,
         "public": false,
         "createdAt": "2024-12-18T23:08:57.419241Z",
         "references": [],
         "metadata": {
            "name": "mywf",
            "organization": "my-org",
            "project": "myproject",
            "team": ""
         }
      },
      {
         "digest": "sha256:ebbaaf0353ae054d6ba24ce1384d7b8244d063c89177bd090a1aeb37dc0af0e6",
         "kind": "ATTESTATION",
         "downloadable": true,
         "public": false,
         "createdAt": "2024-12-18T23:10:13.171466Z",
         "references": [],
         "metadata": {
            "name": "mywf",
            "organization": "my-org",
            "project": "myproject",
            "team": ""
         }
      }
   ]
}