Fix url-decode benchmark to use offsetalator

[davidwendt]

Description

Fixes the logic for generating URLs in the url-decoder benchmark to use the offsetalator instead of hardcoding size_type.
This will allow benchmarking with large strings column in the future.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

/merge