feat: add example contracts

The `stellar contract init` command no longer works with
`--frontend-template` (stellar/stellar-cli#1625)

This means that templates like this one need to hard-code their own list
of contracts.

contracts/hello_world/Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "soroban-hello-world-contract"
+version = "0.0.0"
+edition = "2021"
+publish = false
+
+[lib]
+crate-type = ["cdylib"]
+doctest = false
+
+[dependencies]
+soroban-sdk = { version = "22.0.1" }
+
+[dev-dependencies]
+soroban-sdk = { version = "22.0.1", features = ["testutils"] }
+
+[profile.release]
+opt-level = "z"
+overflow-checks = true
+debug = 0
+strip = "symbols"
+debug-assertions = false
+panic = "abort"
+codegen-units = 1
+lto = true
+
+[profile.release-with-logs]
+inherits = "release"
+debug-assertions = true

contracts/hello_world/Makefile

@@ -0,0 +1,16 @@
+default: build
+
+all: test
+
+test: build
+	cargo test
+
+build:
+	stellar contract build
+	@ls -l target/wasm32-unknown-unknown/release/*.wasm
+
+fmt:
+	cargo fmt --all
+
+clean:
+	cargo clean

contracts/hello_world/src/lib.rs

@@ -0,0 +1,14 @@
+#![no_std]
+use soroban_sdk::{contract, contractimpl, vec, Env, String, Vec};
+
+#[contract]
+pub struct HelloContract;
+
+#[contractimpl]
+impl HelloContract {
+    pub fn hello(env: Env, to: String) -> Vec<String> {
+        vec![&env, String::from_str(&env, "Hello"), to]
+    }
+}
+
+mod test;

contracts/hello_world/src/test.rs

@@ -0,0 +1,21 @@
+#![cfg(test)]
+
+use super::*;
+use soroban_sdk::{vec, Env, String};
+
+#[test]
+fn test() {
+    let env = Env::default();
+    let contract_id = env.register(HelloContract, ());
+    let client = HelloContractClient::new(&env, &contract_id);
+
+    let words = client.hello(&String::from_str(&env, "Dev"));
+    assert_eq!(
+        words,
+        vec![
+            &env,
+            String::from_str(&env, "Hello"),
+            String::from_str(&env, "Dev"),
+        ]
+    );
+}