chore: remove NATS transport support

Remove the `wrpc-nats` transport crate and all NATS integration across
the workspace:

- delete `crates/nats` and the `nats` cargo features/dependencies in the
  `wrpc`, `wrpc-cli`, `wrpc-test` and `wrpc-wasmtime-cli` crates
- drop the `nats` subcommand from `wrpc-wasmtime` and the NATS connect
  helper from `wrpc-cli`
- remove the NATS Rust/Go/web examples and the `go/nats` client
- switch the Go integration tests and the Rust benchmarks from NATS to
  the framed TCP transport
- drop NATS from the Nix dev shell/checks and the CI publish matrix,
  remove the NATS sections from the README and SPEC, and tidy all Go
  modules
- enable the `async-await` feature on the `futures` dev-dependency so
  `futures-macro` stays in the vendored crate set used by the offline
  `wit-bindgen-wrpc test` bindgen check (previously pulled in transitively
  by `async-nats`)

Assisted-by: claude:claude-opus-4-8

Author: rvolosatovs

.github/workflows/wrpc.yml

@@ -11,8 +11,6 @@ on:
       - 'crates/cli/v[0-9].[0-9]+.[0-9]+-*'
       - 'crates/introspect/v[0-9].[0-9]+.[0-9]+'
       - 'crates/introspect/v[0-9].[0-9]+.[0-9]+-*'
-      - 'crates/nats/v[0-9].[0-9]+.[0-9]+'
-      - 'crates/nats/v[0-9].[0-9]+.[0-9]+-*'
       - 'crates/pack/v[0-9].[0-9]+.[0-9]+'
       - 'crates/pack/v[0-9].[0-9]+.[0-9]+-*'
       - 'crates/quic/v[0-9].[0-9]+.[0-9]+'
@@ -211,9 +209,6 @@ jobs:
 
           - crate: introspect
 
-          - crate: nats
-            workspace-dependencies: true
-
           - crate: pack
             workspace-dependencies: true