Add a command to generate self-signed certificates from a private signing key

[paulhowardarm]

Summary

Add a new command to the tool that can output a complete, self-signed X509 certificate based on a private signing key.

Details

The following recent PR introduced the create-csr command, which can create a Certificate Signing Request (CSR) from a private key: #68

The tool uses the rcgen crate to make the CSR.

This crate is also capable of making complete self-signed certs as well as CSRs, and it might be handy to also add this facility to parsec-tool.

Much of the implementation of the existing create-csr command could possibly be copied or refactored in order to output a self-signed cert instead of a CSR. Some of the command-line inputs would also be the same, although there would also be the need for additional inputs because certs have more fields than CSRs have.

Automated tests would also be required in the CLI test script.