Add support for protobuf map fields

Update the generator to detect map fields and use MapFieldIterator.
Implement MapFieldIterator in the runtime to handle key-value pair
extraction and add write_map_entry to ProtoBuilder. Add tests to
verify that map-bearing messages generate and compile correctly.

codegen/tests/test_map_build.rs

@@ -0,0 +1,67 @@
+use roto_codegen::google::protobuf::descriptor::FileDescriptorSet;
+use std::fs;
+use std::process::Command;
+
+#[test]
+fn test_map_generated_code_builds() {
+    // 1. Load FileDescriptorSet from data/test_map.desc
+    let desc_path = "data/test_map.desc";
+    let data = fs::read(desc_path).expect("Failed to read test_map.desc");
+    let set = FileDescriptorSet::new(&data)
+        .expect("Failed to create FileDescriptorSet from test_map.desc");
+
+    let generated_files = roto_codegen::generator::generate_rust_code(&set, None, false);
+    assert!(
+        !generated_files.is_empty(),
+        "Generated code should not be empty"
+    );
+
+    // 2. Setup a temporary Cargo project to verify the code builds
+    let root = std::env::current_dir().expect("Failed to get current directory");
+    let temp_project_dir = root.join("test_map_gen_project");
+
+    // Clean up previous runs
+    if temp_project_dir.exists() {
+        fs::remove_dir_all(&temp_project_dir).expect("Failed to clean up temp project directory");
+    }
+
+    // Create new library project
+    let status = Command::new("cargo")
+        .args(["new", "--lib", "test_map_gen_project"])
+        .current_dir(&root)
+        .status()
+        .expect("Failed to run cargo new");
+    assert!(status.success(), "cargo new failed");
+
+    // 3. Configure the project to depend on the current roto crate
+    let cargo_toml_path = temp_project_dir.join("Cargo.toml");
+    let cargo_toml_content =
+        fs::read_to_string(&cargo_toml_path).expect("Failed to read Cargo.toml");
+    let updated_cargo_toml = format!(
+        "{}\n\nroto-codegen = {{ path = \"..\" }}\nroto-runtime = {{ path = \"../../runtime\" }}\n\n[workspace]\n",
+        cargo_toml_content
+    );
+    fs::write(cargo_toml_path, updated_cargo_toml).expect("Failed to write Cargo.toml");
+
+    // 4. Write the generated code to src/lib.rs
+    let mut all_code = String::new();
+    for (_, content) in generated_files {
+        all_code.push_str(&content);
+        all_code.push_str("\n");
+    }
+    let final_code = all_code.replace("use crate::", "use roto::");
+    let lib_path = temp_project_dir.join("src/lib.rs");
+    fs::write(lib_path, final_code).expect("Failed to write generated code to src/lib.rs");
+
+    // 5. Attempt to build the project
+    let build_status = Command::new("cargo")
+        .args(["--offline", "build"])
+        .current_dir(&temp_project_dir)
+        .status()
+        .expect("Failed to run cargo build");
+
+    assert!(
+        build_status.success(),
+        "The generated Rust code for test_map.proto failed to build in a standalone project!"
+    );
+}