chore: fuel-indexer: refactor run, service, and executor to take advantage of tokio::task::JoinSet

[lostman]

Description

This PR cleans up parts of IndexerService, Executor, and fuel-indexer's run.rs.

There is a slight change of semantics due to the replacement of FuturesUnordered with JoinSet: JoinSet aborts all tasks when it is dropped. This appeared in web-api tests that dropped the IndexerService object before the tests' completion. Tasks added to FuturesUnordered would've been running anyway (since task::spawn detaches the future). Tasks spawned into the task set of IndexerService are aborted. This required a small change to the tests to keep the IndexerService alive.

Testing steps

CI tests should pass.

Changelog

• Replace FuturesUnordered with tokio::task::JoinSet
• Clean up code in service.rs and executor.rs: move the kill switch into Executor implementations and expose through the trait interface; expose the manifest the same way. This allows cleaning up some functions that now only need the executor and no other parameters.
• Simplify fuel-indexer's run.rs to use the JoinSet for all subsystems (node, web-api, signal-handler, etc.).
• WasmIndexExecutor::new() takes WASM bytes and not ExecSource: we were passing ExecSource::Manifest and returning ExecSource::Registry(wasm_bytes). Instead, the WASM bytes are loaded before new() is called, and everything is simpler this way.
• IndexerService::run removes completed tasks from the JoinSet and runs the service loop: loop { tokio::select! ... }. This way, we don't need two separate tasks for it.

Measuring resource usage:

Changes in this PR have no adverse effect on the resource usage:

[deekerno]

This looks pretty good! I like the cleanup. I left a few comments to be addressed before approval.

[deekerno]

🆗