4. `parse_rsa_pem(pem_contents, display_path) ->
Result<EncodingKey,
GitHubError>`: calls `validate_rsa_pem` first, then `EncodingKey::from_rsa_pem(pem_contents.as_bytes())
`, mapping `jsonwebtoken::errors::Error` to `
GitHubError::PrivateKeyLoadFailed`.