Make the SDK able to import keys created by OpenSSL

[petreze]

Problem

Currently the SDK cannot import ECDSA/ED25519 private keys generated by OpenSSL and exporting a public/private key to DER generates different string from OpenSSL

Solution

• Make the SDK compatible with OpenSSL
• Add tests for import/export of ECDSA and ED25519 keys from/to DER
• Add tests for importing ECDSA private key from PEM
• Add example

Alternatives

No response