Feat: add `genesis generate-keypair` command in aggregator

[jpraynaud]

Content

This PR includes the implementation and documentation of the new genesis generate-keypair command in the aggregator that is used to create Genesis keypairs.

Documentation Updates:

• Added generate-keypair command to the list of subcommands in mithril-aggregator.md.
• Documented parameters for the generate-keypair command in mithril-aggregator.md.

Command Implementation:

• Added GenerateKeypair variant to the GenesisSubCommand enum in genesis_command.rs.
• Implemented the execute method for the GenerateKeypairGenesisSubCommand struct in genesis_command.rs.

Utility Functions:

• Added create_and_save_genesis_keypair function to GenesisTools in genesis.rs.
• Updated the export_keypair_to_files method in ProtocolGenesisSigner to export both secret and verification keys in genesis.rs.

Test Updates:

• Added a test for the create_and_save_genesis_keypair function in genesis.rs.
• Modified existing tests to use the new keypair export function in genesis.rs.

Pre-submit checklist

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

Issue(s)

Closes #2074

[github-actions[bot]]

Test Results

    4 files  ±0     51 suites  ±0   11m 9s ⏱️ -17s 1 443 tests +2  1 443 ✅ +2  0 💤 ±0  0 ❌ ±0  1 654 runs  +2  1 654 ✅ +2  0 💤 ±0  0 ❌ ±0 

Results for commit 8b1ec901. ± Comparison against base commit e274a5d9.

:recycle: This comment has been updated with latest results.