Add public key export to pico-hsm-tool.py

[krlnokrl]

Summary

Describe in plain language what this PR does and why.

• Small functionality improvement to pico-hsm-tool

• Add the functionality to export public keys to pico-hsm-tool.py
  This exposes the function public_key() from pypicohsm/PicoHSM.py

• Keys can be written to console or to file in PEM, DER and OpenSSH format.

Testing

• Hardware / board(s) tested: picoHSM 6.4 - pi-pico2
• Library version: pypicohsm version "2.0.2"
• Exported keys tested: RSA, ED25519, EC:secp256r1, EC:secp512r1
• Formats tested: PEM, DER, OpenSSH

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  • under the AGPLv3 Community Edition, and
  • under any proprietary / commercial / Enterprise editions of this project,
    now or in the future.
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• Yes, I agree

Anything else?

Optional: missing official mirror of pypicohsm and pypicokey libraries renders pico-hsm-tool hard to develop.