Fix/java8 signature format

[oleksandrlazarenko-pi]

Summary

Replaces the SHA256withECDSAinP1363Format signature algorithm with the
standard SHA256withECDSA, converting between ASN.1 DER and the raw 64-byte
(IEEE P1363) form in code. The wire format is unchanged; the library now signs
and verifies on Java 8 instead of requiring Java 15+.

Motivation

SHA256withECDSAinP1363Format was only added to the JDK in Java 15. Since
this library targets Java 8 (maven.compiler.release=8), any consumer on Java
8-14 hit NoSuchAlgorithmException: SHA256withECDSAinP1363Format Signature not available the moment they signed or verified — which surfaced as CI failures in
a downstream Java 8 project:

com.swancommunity.owid.OwidException: key operation failed because
  SHA256withECDSAinP1363Format Signature not available
Caused by: java.security.NoSuchAlgorithmException

So the Java-8 compile target was misleading: the crypto still needed Java 15+.
The PHP implementation already avoids this (it uses SHA256withECDSA +
derToRaw/rawToDer); this change brings owid-java in line.

Changes (Crypto.java)

• SIGNATURE_ALGORITHM → SHA256withECDSA (DER; available since Java 7).
• signByteArray converts the DER signature to the raw 64-byte form via a new
  derToRaw; verifyByteArray converts the raw 64 bytes back to DER via
  rawToDer before verifying.
• Added minimal, dependency-free ASN.1 helpers (derToRaw, rawToDer,
  readDerLength, readDerInteger, padCoordinate, encodeDerInteger,
  encodeDerLength, trimLeadingZeros) — all pure byte manipulation, Java 8 safe.

Compatibility

• Wire format unchanged — the emitted signature is still the raw 32-byte r
  followed by the 32-byte s. The existing WireVectorsTest and FixturesTest
  (cross-language vectors) pass unchanged, confirming byte-for-byte interop with
  owid-js / owid-php / owid-python / owid-dotnet.
• API unchanged — no public method signatures change.

Testing

• owid-java: mvn clean test — 35/35 pass (incl. CryptoTest sign/verify
  round-trips and the wire-format fixtures).
• Downstream Java 8 reader: 32/32 pass, with an animal-sniffer java18 API
  check confirming only Java 8 APIs are used.