Add ES256K support

[imirkin]

Unfortunately it doesn't seem like RFC 8812 provides any "worked" examples, unlike some of the other RFCs. This roundtrips successfully with the implementation I recently added to github.com/lestrrat-go/jwx.

There's also a libsecp256k1 library from the bitcoin project (https://github.com/bitcoin-core/secp256k1). However it doesn't seem to have a stable release, and it's unclear that the extra dependency would be worth it over openssl for this usage.

If this is accepted, I'd be happy to add EdDSA and x25519/x448 curve support.

[sarroutbi]

Hello @imirkin. Could you please try to rebase master branch to this one? This would allow to retrieve changes with the newest CI strategy (based on Github action), so that it could be checked if checks pass on your PR?

[codecov-commenter]

Codecov Report

Merging #90 (63d28ab) into master (4248b93) will decrease coverage by 1.16%. The diff coverage is 94.11%.

:exclamation: Current head 63d28ab differs from pull request most recent head c43d553. Consider uploading reports for the commit c43d553 to get more accurate results

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   77.58%   76.41%   -1.17%     
==========================================
  Files          60       57       -3     
  Lines        5648     5322     -326     
==========================================
- Hits         4382     4067     -315     
+ Misses       1266     1255      -11     

Impacted Files	Coverage Δ
lib/openssl/ecdsa.c	80.89% <91.66%> (-0.69%)	:arrow_down:
lib/openssl/ec.c	80.64% <100.00%> (ø)
lib/openssl/jwk.c	69.10% <100.00%> (-1.10%)	:arrow_down:
cmd/jws/jws.h	76.47% <0.00%> (-11.41%)	:arrow_down:
tests/api_jwe.c	92.85% <0.00%> (-3.92%)	:arrow_down:
cmd/jwe/enc.c	41.42% <0.00%> (-2.41%)	:arrow_down:
cmd/jws/sig.c	65.85% <0.00%> (-2.33%)	:arrow_down:
cmd/jwe/dec.c	51.85% <0.00%> (-2.27%)	:arrow_down:
lib/openssl/ecmr.c	75.43% <0.00%> (-1.62%)	:arrow_down:
lib/cfg.c	39.47% <0.00%> (-1.56%)	:arrow_down:
... and 37 more

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[sarroutbi]

@sergio-correia : In my opinion, this looks good. Please, provide your feedback when possible

[sergio-correia]

Looks good to me, thanks.