Opt-in ES256K (RFC 8812) package with a secp256k1 curve #337

ES256K (RFC 8812, ECDSA over secp256k1 with SHA-256) is registered in the IANA JOSE algorithms registry but not supported here.

Ship it as an opt-in package that registers the secp256k1 EccCurveSpecification (ADR-0068) and an EccSignatureAlgorithm bound to that curve. #331 binds each ES* algorithm to its curve, which is what makes this safe: a P-256 key cannot sign as ES256K, and a secp256k1 key cannot sign as ES256. Gate registration on platform support, because .NET cannot use secp256k1 on every operating system (macOS in particular). Part of the coverage tracked in #66.

Not needed for the first deployment.