Hardware Security Module (HSM) Support

[shibumi]

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Description of issue or feature request:
For supporting Hardware Security Modules (HSMs), we need the the following features:

• possibility to pass a custom keyID
• possibly more?

Current behavior:

Currently, we have no HSM support, because we load keys from file. HSM support is especially interesting in combination with a smartcard daemon (for example a RSA key on a Yubikey).

Expected behavior:

We should support HSMs, if we want to keep up with modern security options.

[lukpueh]

Consult with secure-systems-lab/securesystemslib#229