Adding Mithril networks configuration to `networks.json`

[dlachaume]

Content

This PR includes the mithril-networks configuration for the Cardano networks (mainnet, preprod, preview, and sanchonet) in the networks.json file.

Pre-submit checklist

• Branch
  • [ ] Tests are provided (if possible)
  • [ ] Crates versions are updated (if relevant)
  • [ ] 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 README file (if relevant)
  • [ ] Update documentation website (if relevant)
  • [ ] Add dev blog post (if relevant)

Issue(s)

Closes #1638

[github-actions[bot]]

Test Results

    3 files  ±0     43 suites  ±0   8m 24s :stopwatch: +10s   976 tests ±0    976 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  1 070 runs  ±0  1 070 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit e9ae3c5b. ± Comparison against base commit cf84a951.

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