Add support for PEM encoded keys

[mat813]

In a project I have, I need to input the private key in a GitLab protected CI/CD pipeline variable, and in GitLab, variables are entered in a web text input field, so copy and pasting the current "binary" key file is not working.

I thought of converting it to base64, and then converting it back in my pipeline but it was kind of kludgy, and it then hit me that is what the PEM format is all about, having a simple header/footer, and the encoded content in base64 inside.

I don't think I changed the zipsign-api public API, so it should not break any existing code when upgrading.

A quick cargo build -r on main and on the pem branch tells me this adds ~21kB to the resulting cli:

-rwxrwxr-x 2 mat mat 1041032 Mar 28 19:37 target/release/zipsign
-rwxrwxr-x 2 mat mat 1062056 Mar 28 19:37 target/release/zipsign

[Kijewski]

Thank you, I like it, seems like a good feature to me! I'll review your PR on Monday.

[mat813]

fixed fmt