feat(policies): expose signing options in attestation

[jiparis]

This PR adds signing options to the statement predicate. This is convenient to create policies that check for signing configurations (regardless of the final signing method used for the attestation).
New properties are:

• signingCA (name of the CA provider configured)
• signingTSA (TSA URL)

This is a best effort to detect best practices at crafting time.

> chainloop wf run describe --id 7d836fc3-287d-46ec-82ca-1c469324e866 -o statement
WRN API contacted in insecure mode
{
   "type": "https://in-toto.io/Statement/v1",
   "subject": [
      {
         "name": "chainloop.workflow.mywf",
         "digest": {
            "sha256": "8a33f95f7deb814a5515a5e8b1bbcd3a18ae341e000e55793f1b121321add447"
         }
      },
      {
         "name": "git.head",
         "digest": {
            "sha1": "1eead8a0bbe0bad68577c4993b2fb17da5f7162c"
         },
        ...
   ],
   "predicate_type": "chainloop.dev/attestation/v0.2",
   "predicate": {
      "buildType": "chainloop.dev/workflowrun/v0.1",
      "builder": {
         "id": "chainloop.dev/cli/dev@sha256:18af0c5f7fe44f6a0c704902b1be82d7dd6865a64035313cf9b7e9e352d3a1c5"
      },
      "metadata": {
          ...
      },
     ...

      "signingCA": "fileCA"
   }
}