Add support for Ed25519KeyPair::generate

[skmcgrail]

Issues:

Resolves #572

Adds support for Ed25519KeyPair::generate(). I've opted to not allow passing a reference to an rng like that presented in the issue, as the parameter would be ignored like it is in the other generation functions available today.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.62%. Comparing base (c358484) to head (367de83).
Report is 104 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/ed25519.rs	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
- Coverage   95.80%   92.62%   -3.18%     
==========================================
  Files          61       67       +6     
  Lines        8143     9393    +1250     
  Branches        0     9393    +9393     
==========================================
+ Hits         7801     8700     +899     
- Misses        342      421      +79     
- Partials        0      272     +272     

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