Expose Ed25519PublicKey

[justsmth]

Description of changes:

• Exposes the Ed25519PublicKey type.

Call-outs:

• We already expose both the EcdsaPublicKey and RsaSubjectPublicKey types, but had not done so for Ed25519.
• We have internal an test for AsDer::<PublicKeyX509Der> on Ed25519PublicKey.

Testing:

• Updated our testing to better cover this API.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 92.89%. Comparing base (c358484) to head (f87232e).
Report is 167 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #695      +/-   ##
==========================================
- Coverage   95.80%   92.89%   -2.91%     
==========================================
  Files          61       69       +8     
  Lines        8143     9565    +1422     
  Branches        0     9565    +9565     
==========================================
+ Hits         7801     8885    +1084     
- Misses        342      399      +57     
- Partials        0      281     +281     

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