fix: ECDSA key pair generation and related documentation

[darkgl0w]

Hello.

• Fixes tests by generating an ES256 (used curve: prime256v1) key pair, using a secp256k1 curve generates an ES256K key pair (currently not supported by jsonwebtoken or fast-jwt in the upcoming release).
• Updates the related documentation.

For reference: rfc8812 - section 3.2.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[mcollina]

cc @simoneb

[simoneb]

/cc @radomird

[radomird]

Yes, looks good to me too. Thanks.

[darkgl0w]

Right now new key pairs are automatically generated by the helper each time the tests will be run ^^

[radomird]

Right now new key pairs are automatically generated by the helper each time the tests will be run ^^

Yes, true - I've deleted the question after realising that we do that. We don't do that in fast-jwt (that is why i was confused for a bit there). In fast-jwt we have pre-generated keys.