Allow String keys to be set for error messages

https://github.com/luckyframework/avram/blob/a604962a50f630d58b85c88e1efd5e73fbdb09f8/src/avram/operation_errors.cr#L19

I have a case where I get errors back from an API JSON. I want to pass those along in the error messages, but I can't just `add_error(err["key"].as_s, err["val"].as_s)`, and there's no way to convert JSON to Symbol, or a String to a Symbol.

The big issue here is that it becomes a major breaking change to allow both Symbol and String. I'm not sure what the solution is here, but we should definitely make this more flexible.

Start at src/avram/operation_errors.cr#L19 and inspect how error keys are currently accepted and stored. Determine a compatible way to support String keys alongside existing Symbol callers, then verify that API JSON-derived keys and current error-message behavior both work without breaking existing usage.

Written by the indexing model from the issue text.