UIHandler: extract a public helper to process custom interactions

[jiripudil]

resolves #388

Food for thought: this change makes it possible to add method, url, and data to the interaction event. Is there a use case for that? Is there a use case for allowing the interaction event listeners to change any of those?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.05%. Comparing base (d4c1132) to head (a176bb3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #395 +/- ## ========================================== + Coverage 93.50% 94.05% +0.54% ========================================== Files 11 11 Lines 1294 1295 +1 Branches 180 176 -4 ========================================== + Hits 1210 1218 +8 + Misses 82 75 -7 Partials 2 2 ```

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