Migrate JCA signing from DER encoding to P1363 encoding

[iaik-jheher]

DER-encoded ECDSA signatures are an ASN.1 structure, with variable length integer representation. P1363-encoded ECDSA signatures are fixed-length concatenated integers with no additional structure.

This means that P1363 -> DER is easy, while DER -> P1363 is impossible without additional information (such as the curve used).
We currently use the RawByteEncodable marker interface to indicate whether a given (EC) signature can be encoded to P1363.

The JCA extensions in Indispensable currently produce and consume DER signatures. This is awkward, since it means that CryptoSignature.parseFromJCA cannot return RawByteEncodable signatures -- not even in the RSA case!

This PR fixes this. It declares the individual parts of the signing process as hazmat (since they are encoding dependent) and switches them to use P1363. The stable API going forward for signing ops using the JCA will be .signWithJCA, which guarantees that the signature object and signature byte decoding use the same encoding (whatever that may be on any given platform).

[JesusMcCloud]

so… do you want me to add a dedicated android target? if you fear duplicated code, we can just add a jvmAndroidMain source folder and hook it into the JVM and Android source sets, so everything but the culprit can be shared and we require no code duplication