Public signature fixtures

The DER certificate and ECDSA signatures cover the fixed message Pocket Radio public crypto adapter test using P-384 with SHA-256 and SHA-384. They were generated with Python cryptography. The private key is not stored.

These public fixtures are included only with the crypto-self-test firmware feature. They check valid signatures, altered messages/signatures, and malformed certificates on the device.

The extended tests also use the existing public P-256 fixture in radio-webrtc/tests/fixtures/ to check native signing and verification. That public test key is never used for the deployed radio's session identity.