Adds ability to specify KeyId to SSH Cert

[EthanHeilman]

Prior to this PR the key id of the SSH certificate was always chosen to be a random value. This PR changes this so that the key ID (a.k.a. the Certificate ID) can be specified by the user in the SigningArguments passed SignCert. This PR maintains backwards compatibility, if no key ID is specified to defaults to the old behavior.

By allowing the key ID to specified in the Signing Argument, the key ID can be associated with the user's identity as intended to OpenSSH.

"key id is a free-form text field that is filled in by the CA at the time of signing; the intention is that the contents of this field are used to identify the identity principal in log messages." -OpenSSH Certificate Spec

This PR:

• Updates SigningArguments struct to take key id. The key id is then included in the signed certificate
• A unit test tests to verify this new behavior, checks that backwards compatibility is maintained and improves coverage on the other fields of the SigningArguments
• Fixes minor bug where we didn't catch an error if base64 decoding failed.
• Updates the go version to 1.22.4 and adds a go.mod file. Before this PR the go version was old enough that it didn't have a go.mod file.