Add common crate to access env variables

In the future, we should use a proper secret store (not just env variables).
This lets us update one block of code and not n in the future.

common/env/src/lib.rs

@@ -0,0 +1,8 @@
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+
+// Obtain a variable from the Serai environment/secret store.
+pub fn var(variable: &str) -> Option<String> {
+  // TODO: Move this to Kubernetes
+  std::env::var(variable).ok()
+}