feat: add elasticsearch tracing subscriber

create lib/pk_core

lib/pk_core/src/util.rs

@@ -0,0 +1,7 @@
+/// get_env gets an env variable as Option<String instead of Result<String>.
+pub fn get_env(key: &str) -> Option<String> {
+    match std::env::var(key) {
+        Ok(val) => { Some(val) }
+        Err(_) => None
+    }
+}