feat: add SnapStart support via SnapStartResource trait

[bnusunny]

Summary

Adds AWS Lambda SnapStart support to the Rust runtime. SnapStart reduces cold-start latency by snapshotting the initialized execution environment and restoring from it, but resources created during init (connections, credentials, unique values) may be invalid after restore. This PR gives functions a place to release/rebuild them, and makes the runtime itself restore-safe.

What's added

SnapStartResource trait (lambda-runtime/src/snapstart.rs) — a CRaC-style trait for custom snapshot/restore logic:

pub trait SnapStartResource: Send + Sync {
    fn before_snapshot(&self) -> BoxFuture<'_, Result<(), Error>> { /* no-op default */ }
    fn after_restore(&self)   -> BoxFuture<'_, Result<(), Error>> { /* no-op default */ }
}

• Returns BoxFuture (not async fn) so the trait stays object-safe as dyn SnapStartResource with no async-trait dependency.
• Both methods default to no-ops; register instances via Runtime::register_snapstart_resource(Arc<...>).

Lifecycle the runtime drives (from both run() and run_concurrent()):

before_snapshot hooks (LIFO) -> GET /restore/next (blocks until restore)
  -> rebuild RAPID connection pool -> after_restore hooks (FIFO)

Register foundations first; teardown runs reverse (LIFO), rebuild forward (FIFO). When SnapStart is not enabled (AWS_LAMBDA_INITIALIZATION_TYPE != "snap-start"), hooks are never invoked.

Error handling — a before_snapshot//restore/next failure is reported to POST /init/error; an after_restore failure to POST /restore/error. Both propagate out via the normal Result path (no process::exit, so graceful-shutdown handlers still run).

Non-breaking connection-pool reset — new PooledClient (lock-free OnceLock-based reset_pool()) and a RuntimeApiClient trait in the api-client crate. The service layer takes a defaulted generic C = Client bounded by the trait and uses PooledClient internally. The old Client is frozen at its 1.0.3 shape and superseded. cargo-semver-checks confirms all three crates stay non-breaking (api-client is a 1.1.0 minor bump).

Changes

• lambda-runtime-api-client: add PooledClient + RuntimeApiClient trait + PooledClient::builder; supersede Client/build; bump to 1.1.0
• lambda-runtime: add SnapStartResource trait, Init/Restore error requests, Runtime::is_snapstart() and register_snapstart_resource(); run the restore lifecycle; re-export SnapStartResource and BoxFuture
• lambda-http: add runtime()/runtime_concurrent() and streaming_runtime() helpers; re-export SnapStartResource and BoxFuture
• lambda-extension: use PooledClient
• examples: add basic-snapstart and http-snapstart (with Dockerfiles on provided:al2023)

Testing

Unit tests cover request serialization, the full restore lifecycle, LIFO/FIFO ordering, no-op when not SnapStart, and the before/after/restore-next failure paths.