chore: rollback binary encoding for single signature

[jpraynaud]

Content

This PR includes the rollback of the binary encoding for the single signature to the already used JSON hex encoding.
This feature is not mature enough at this time, and releasing it with the next distribution would not allow to evolve this encoding without a Mithril era.

Pre-submit checklist

• Branch
  • Crates versions are updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
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Issue(s)

Relates to #2625

[github-actions]

Test Results

    4 files  ±0    164 suites  ±0   24m 1s ⏱️ +13s
2 181 tests ±0  2 181 ✅ ±0  0 💤 ±0  0 ❌ ±0 
6 706 runs  ±0  6 706 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 4671b53. ± Comparison against base commit 9d9e963.

[Copilot]

Pull Request Overview

This PR reverts the binary encoding for single signatures back to JSON hex encoding, as the binary encoding feature is not yet mature enough for production release. The rollback ensures that the encoding can evolve without requiring a Mithril era change.

• Moved SingleSignature from bytes_hex_codec to json_hex_codec implementation
• Updated crate version from 0.6.15 to 0.6.16

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
mithril-common/src/crypto_helper/types/wrappers.rs	Reverted SingleSignature encoding from binary to JSON hex
mithril-common/Cargo.toml	Bumped version to 0.6.16

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[turmelclem]

LGTM 👍