futures: Service abstraction

## Description

An abstraction for coordinating the lifecycle of tokio tasks, looking
after:

- Building up services by adding tasks or through composition.
- Waiting for multiple tasks to complete.
- Running secondary tasks that need to be wound down when the service
  winds down, but otherwise don't affect the lifecycle of the service
  (the service doesn't wait for these tasks to complete to shutdown).
- Graceful shutdown, with a grace period, because the service has
  wrapped up, or because of a termination request, or an error.
- Panic unwinding.
- Cascading termination (errors from multiple components, multiple
  termination requests, timeout during shutdown).
- Signal handling from the operating system.

## Test plan

New unit tests:

```
sui-futures$ cargo nextest run -- service
```

Author: amnn

Cargo.lock

@@ -7,8 +7,12 @@ publish = false
 edition = "2024"
 
 [dependencies]
+anyhow.workspace = true
 futures.workspace = true
+tap.workspace = true
+thiserror.workspace = true
 tokio = { workspace = true, features = ["rt", "time", "macros"] }
+tracing.workspace = true
 
 [dev-dependencies]
-tokio = { workspace = true, features = ["full"] }
+tokio = { workspace = true, features = ["full", "test-util"] }

crates/sui-futures/Cargo.toml

@@ -2,4 +2,5 @@
 // SPDX-License-Identifier: Apache-2.0
 
 pub mod future;
+pub mod service;
 pub mod stream;