Warns missing `handle_input/3` on `request_input/2`

[adkron]

Description

I didn’t want development workflows or older projects to break, so I thought a warning would be an excellent solution to reduce the debugging time but still allow current projects to continue working with this enhancement. This means it is still possible to implement the wrong callback, but hopefully, the warning will point the developer in the right direction and save minutes, if not hours, of debugging.

Motivation and Context

With all the different handlers for events, inputs, and messages, it can be simple to pick the wrong one when implementing. This leads to bugs that are difficult to find.

Types of changes

• [x] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist

• [x] Check other PRs and make sure that the changes are not done yet.
• [x] The PR title is no longer than 64 characters.