Rename fields to match types

[natalieesk]

This closes #495

1. Updated SecretShare, Package and KeyPackage fields to use signing_share
2. Updated PublicKeyPackage to use verifying_shares
3. Updated KeyPackage to use verifying_key
4. Removed secret() from SecretShare

[github-actions]

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02% ⚠️

Comparison is base (c38305b) 81.18% compared to head (0356cab) 81.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #539      +/-   ##
==========================================
- Coverage   81.18%   81.17%   -0.02%     
==========================================
  Files          30       30              
  Lines        3014     3012       -2     
==========================================
- Hits         2447     2445       -2     
  Misses        567      567              

Files Changed	Coverage Δ
frost-core/src/frost.rs	92.36% <100.00%> (ø)
frost-core/src/frost/keys.rs	98.42% <100.00%> (-0.02%)	⬇️
frost-core/src/frost/keys/dkg.rs	89.13% <100.00%> (+0.04%)	⬆️
frost-core/src/frost/keys/repairable.rs	97.14% <100.00%> (ø)
frost-core/src/frost/round2.rs	100.00% <100.00%> (ø)
frost-rerandomized/src/lib.rs	83.62% <100.00%> (ø)

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[conradoplg]

This is great, thank you!

Could you please add a CHANGELOG entry for this, listing the old and new names, e.g. "SecretShare::value()" was changed to SecretShare::signing_share()"?

I'm approving this so it will get merged soon, to unblock other work, you can do that in a separate PR if it's OK.