cmd: support private keys in `kes identity`

[aead]

This commit is part of the CLI refactoring https://github.com/minio/kes/milestone/6

This commit adds support for parsing PEM private key files and printing their private key as API key.

Now, the kes identity command is now able to generate API keys, print identities of API keys or certificates and convert private key files to API keys. The existing sub-commands new, of, info ls are still supported.

As of now, only private keys of type Ed25519 are supported.