Gate the server side behind a default server feature #18

The crate has no Cargo features, so a wasm guest that wants only the messages and the request/response/resource helpers cannot depend on it: tokio (rt-multi-thread, signal) and aws-lc-sys do not build for wasm32-wasip3.
A server feature, on by default, gating server.rs, metrics.rs, logging.rs, the server-side Error variants, the generated tonic client/server and FILE_DESCRIPTOR_SET keeps every current dependent as is, and with default-features = false the rest compiles for wasm (prototyped against function-wasm's rust scaffold: it builds, passes the guest suite and drops the vendored proto and protoc).