Add an era subcommand to create keypair

[sfauvel]

Content

Creation of a command on the aggregator to create test era keys for setting up a test

This PR includes:

• a new sub-command generate-keypair for the era command to export non deterministic era.sk and era.vk files.
• a parameter target-path to define the export directory
• the update of the aggregator documentation

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is 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 #2271

[github-actions]

Test Results

    4 files  ±0     56 suites  ±0   10m 22s ⏱️ -5s
1 591 tests +1  1 591 ✅ +1  0 💤 ±0  0 ❌ ±0 
1 889 runs  +1  1 889 ✅ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 257cb8f. ± Comparison against base commit b4ad474.

♻️ This comment has been updated with latest results.

[dlachaume]

LGTM 👍

[jpraynaud]

LGTM 👍