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/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "serai-env"
+version = "0.1.0"
+description = "A common library for Serai apps to access environment variables"
+license = "AGPL-3.0-only"
+repository = "https://github.com/serai-dex/serai/tree/develop/common/env"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+keywords = []
+edition = "2021"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]

common/env/LICENSE

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+AGPL-3.0-only license
+
+Copyright (c) 2023 Luke Parker
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License Version 3 as
+published by the Free Software Foundation.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.

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()
+}