refactor: add must_use to event loop handles

[domodwyer]

This helps catch accidental misuse of the event loop subscription API, wherein a subscription is created and then immediately removed due to the returned handle not being retained.

It causes a compiler warning to be emitted that looks like this:

warning: unused `EspSubscription` that must be used
  --> src/wifi.rs:56:9
   |
56 | /         event_loop
57 | |             .subscribe::<WifiEvent, _>({
...  |
90 | |             })
91 | |             .expect("failed to subscribe to event bus");
   | |_______________________________________________________^
   |
   = note: Event subscription is unregistered when handle is dropped
   = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
   |
56 |         let _ = event_loop
   |         +++++++

Submission Checklist 📝

• [N/A] I have updated existing examples or added new ones (if applicable).
• I have used cargo fmt command to ensure that all changed code is formatted correctly.
• I have used cargo clippy command to ensure that all changed code passes latest Clippy nightly lints.
• My changes were added to the CHANGELOG.md in the proper section.

[ivmarkov]

I wonder why you need that one given that it is an internal type? Just for the esp-idf-svc code itself, or am I missing something?

[domodwyer]

I was just about to open the PR with the EspSubscription annoation and I thought "I'll quickly check if there's any other Drop impls" and so stuck one on this - didn't even notice it wasn't pub.

I've pushed 717c0fb to remove this one 👍