chore(deps): clean up unused imports and dependencies in reth-downloaders

[Fallengirl]

Some imports in test_utils/mod.rs were unconditionally imported but only used when the "file-client" feature is enabled or during testing. This adds proper cfg gates to ensure they're only compiled when actually needed.

Changes:

1. Added cfg attributes for feature-gated imports (futures, tokio, std::io)
2. Added explicit tempfile crate usage to satisfy unused_crate_dependencies lint
3. Added alloy-rlp and itertools to dev-dependencies for test compilation

[mattsse]

will allow if we can move the feature gated imports into the functions that require them

[Fallengirl]

moved into the function, thanks

[Fallengirl]

@mattsse pinging