2.1.0

Feat: add optional signed updates support for UUPD 2.1 alpha

Add opt-in cryptographic signing and verification for UUPD releases using
Ed25519 signatures and SHA-256 package validation.

Add optional signed update support with full backwards compatibility
Add Ed25519 public-key verification for update metadata
Fail closed when signed updates are enabled, and metadata is unsigned,
malformed, or fails signature verification
Preserve existing UUPD index.json metadata structure and append signing
fields for compatibility with existing consumers
Add SHA-256 package hashes to authenticated release metadata
Verify downloaded ZIP packages against the signed SHA-256 before install
Add signing key ID support for identifying release keys
Keep private signing keys entirely outside distributed plugin code
Add release signing helper with Ed25519 key-pair generation
Support signing keys supplied from a protected file, deploy.cfg, or
interactively via stdin
Add signed-release support to deploy.sh
Ensure interactive private keys are not written to disk or passed as
command-line arguments
Retain existing unsigned update behaviour when signing is disabled
Cache signature verification failures to avoid repeated failed requests
Add documentation for key generation, configuration, deployment,
verification, and key management