Make the output of export-public-key more useful.

[paulhowardarm]

Summary

The export-public-key command writes to standard output as space-separated hex digits, which are quite difficult to process. We should make it possible to generate PEM, which is the same data in base64 with standard (BEGIN and END) headers and footers - the output can then be trivially copy/pasted into other tools that process keys.

Details

Default behaviour is probably fine as it is, but it would be good to accept a --pem switch on the command line and output as per this example:

-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEAui/eDjvMavCdat/JDTt2/e51eRttNpdAV9mxaEqF8uf4oZYiM0oVPq4DG6pN2FSOgOR9urD57dGG3ddFmUif1cQc8OeHBCB58HwQzNvSAKaB1TU2YFTGjvX5cDw4pT2gCrbzdQGnVAuHog1Mez5iOt/Ken2dN3zQ3LJhOnml08OTr3+909mU9Bf9uTi6ctVznhaqSvHY7ilHzNZfCi/9skwEze+ExA3xwBMeqJ+kOghgozNcLvv+qEWpyfAEh0JtdtzvWvQPqw3bD9jvlnPQ6HoP5xggLEsNcyuFUHKNU03xNK9qsGItC4GMvtc1U6aHLPyZc51y/L3+/8e/HLcJAQIDAQAB
-----END RSA PUBLIC KEY-----

The data bytes are identical to the current output, just base64 encoded, and the headers and footers (for RSA public keys) should be as shown.

It should be possible to paste this output into an encryption tool such as this one, and successfully encrypt a plaintext message: https://8gwifi.org/rsafunctions.jsp

It would be fine to only support this for RSA public keys for now, although we can research how this would scale to other key types by peeking the key properties before getting the raw bytes.