Add signing capabilities to aggregator root route

[dlachaume]

Content

This PR includes an update of the Aggregator root route response /.

Signed entity types signed by the aggregator are now listed in a new field capabilities.

{
  "open_api_version": "0.1.17",
  "documentation_url": "https://mithril.network",
  "capabilities": {
    "signed_entity_types": [
      "MithrilStakeDistribution",
      "CardanoImmutableFilesFull",
      "CardanoTransactions"
    ]
  }
}

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • No clippy warnings in the CI
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)

Issue(s)

Closes #1496

[github-actions]

Test Results

  3 files   40 suites   9m 18s ⏱️
830 tests 830 ✅ 0 💤 0 ❌
917 runs  917 ✅ 0 💤 0 ❌

Results for commit 2c60e8a.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍

I left minor comments below.