Introduce alloc feature for optional allocation

Wrap heap-allocated types and service generation in the `alloc` feature
flag to support environments without a memory allocator.

examples/hello_world/Cargo.toml

@@ -24,7 +24,12 @@ futures-util = "0.3"
 http-body-util = "0.1"
 http = "1.1"
 http-body = "1.0"
+async-trait = "0.1"
 
 [build-dependencies]
 tonic-build = "0.12"
 roto-codegen = { path = "../../codegen" }
+
+[features]
+default = ["alloc"]
+alloc = []