Providers/Base: Add accumulating sign/verify helpers

[ariawisp]

What

• Add AccumulatingSignFunction and AccumulatingVerifyFunction that buffer update() calls and delegate to provided one‑shot operations.
• Implementation mirrors AccumulatingCipherFunction: single ByteArray accumulator and isClosed state.
• Scope: cryptography-providers/base only; no provider wiring or tests changed.
• Add shared helper unwrapSubjectPublicKeyInfo(algorithms: Set<ObjectIdentifier>, bytes: ByteArray) for provider key decoding.

Why

• Avoid duplicate buffering logic across providers; makes streaming sign/verify trivial to implement (e.g., CryptoKit EdDSA).

Follow‑ups

• Providers can adopt these helpers where streaming variants are needed; no API/behavior change until used by providers.
• JDK PR will drop its local helper and call the shared unwrapSubjectPublicKeyInfo(Set, bytes).