refactor: Replaces ed25519-donna source by Conan package

[bthomee]

High Level Overview of Change

This change substitutes the ed25519-donna source code copy by the Conan recipe added in XRPLF/conan-center-index#23.

Context of Change

There was no Conan package for ed25519-donna, so its source code was added to the external/ directory and compiled as part of rippled. As we recently created a package for it in our own Conan Center Index fork, we can now switch to using that package.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.6%. Comparing base (f399749) to head (fc57da9).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6088     +/-   ##
=========================================
- Coverage     78.6%   78.6%   -0.0%     
=========================================
  Files          818     818             
  Lines        68938   68938             
  Branches      8242    8241      -1     
=========================================
- Hits         54178   54162     -16     
- Misses       14760   14776     +16     

see 5 files with indirect coverage changes

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