feat: Implement way of retrieving CAs in signing process

src/main/java/de/mlessmann/certassist/openssl/CertificateUsage.java

@@ -0,0 +1,17 @@
+package de.mlessmann.certassist.openssl;
+
+import java.nio.file.Path;
+
+/**
+ * Instance of a certificate that is temporarily stored on disk to be available for use in command line calls.
+ * The instance implements AutoCloseable to enable cleanup after the stored files are no longer needed.
+ * @implSpec The files should be removed from disk when the instance is closed, UNLESS the provided paths are the permanent storage location for the certificate files.
+ */
+public interface CertificateUsage extends AutoCloseable {
+    Path certificatePath();
+    Path certificateKeyPath();
+    String fingerprint();
+
+    @Override
+    void close();
+}