feat: store authentication identifier in attestation

[migmartri]

This task involves storing in the attestation information what kind of authentication was used and its identifier.

Today, an attestation process can be authenticated via user token, API tokens, or federated tokens (OIDC token in GitLab)

I could think of

type	identifier
USER	UserID
API_TOKEN	TokenID
FEDERATED_GITLAB_TOKEN	tokenID

note that this information should be inside the claims of the JWT tokens. For example, see below an example of a user token

or a Chainloop API token

Once we have this information, it could be rendered in the attestation json file maybe in a auth json message in the predicate, for example

{
  "predicate": {
    "auth": {
      "kind": "USER"
      "id": "deadbeef"
    } 
}

What do you think?

[migmartri]

cc/ @jiparis @javirln