Add Author Field to Governance Action Submissions in Proposal Discussion Forum

### Description
Update the submission process for all Governance Action types in the Proposal Discussion Forum pillar of GovTool to include metadata-compliant author information. This should be implemented as part of the CIP-108 metadata structure.

The submission form should allow the user to enter a name, which will be used as the `name` field in the `authors` array. The corresponding `witness` object (including `witnessAlgorithm`, `publicKey`, and `signature`) should be automatically generated by the PDF pillar.

Refer to the example metadata structure from CIP-108 for a treasury withdrawal governance action:
https://github.com/cardano-foundation/CIPs/blob/master/CIP-0108/examples/treasury-withdrawal.jsonld#L63C1-L72C4

### Acceptance Criteria
- [ ] Submission form includes an input field for author `name`.
- [ ] `authors` block in the resulting metadata includes an array of authors.
- [ ] Each author object includes:
- `name` (from form input)
- `witness` object containing:
- `witnessAlgorithm`
- `publicKey`
- `signature`
- [ ] `witness` values are generated by the PDF pillar automatically.
- [ ] Final JSON-LD metadata conforms to CIP-108 structure.
- [ ] Author information is visible on the governance action submission review page.