refactor sign to bytes in / bytes out and finish out Secp256k1 im…

[mistermoe]

TODO: Fill out

• refactored sign to be bytes in / bytes out
• finished out Secp256k1 implementation

[codecov-commenter]

Codecov Report

Merging #61 (26ae727) into main (20ee82c) will decrease coverage by 1.58%.
The diff coverage is 4.54%.

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   14.66%   13.09%   -1.58%     
==========================================
  Files          15       15              
  Lines         409      443      +34     
  Branches       51       54       +3     
==========================================
- Hits           60       58       -2     
- Misses        346      380      +34     
- Partials        3        5       +2     

Files	Coverage Δ
...to/src/main/kotlin/web5/sdk/crypto/KeyGenerator.kt	0.00% <ø> (ø)
crypto/src/main/kotlin/web5/sdk/crypto/Signer.kt	0.00% <0.00%> (ø)
dids/src/main/kotlin/web5/sdk/dids/DidKey.kt	79.48% <40.00%> (-9.71%)	⬇️
crypto/src/main/kotlin/web5/sdk/crypto/Ed25519.kt	0.00% <0.00%> (ø)
crypto/src/main/kotlin/web5/sdk/crypto/Crypto.kt	0.00% <0.00%> (ø)
...rypto/src/main/kotlin/web5/sdk/crypto/Secp256k1.kt	0.00% <0.00%> (ø)